New 4-Stroke or old 2-Stroke

The problem with four strokes getting 'better' is that they're getting lighter and lighter and small displacement wise and are built with less and less metal. From what I've seen they've become somewhat fragile.
The earlier four strokes seem more robust durability wise.
I've been looking around lately for a 10hp four stroke for use on one lake and motors are either too expensive or worn out. I've looked at over 20 of them and only one had decent compression, four of them had spark plugs that wouldn't come out.
My 16ft boat came with a four year old four stroke Suzuki. After four months of repeated carb issues, a frozen tilt pin, a starter, and lower unit seals and finally a dropped valve I gave up on it and started looking for a good motor. Every used four stroke I looked at had issues. If it wasn't low compression it was gas in the oil, rusty parts or they were flat out parts motors at best. I gave up on four strokes after coming to the realization that they don't last as long as a good two stroke. Four different dealers and shops all made it clear that the best motors were made pre 1992. When I started looking at older motors I started finding more that had excellent compression and parts were cheaper as well.

I bought a 1988 50hp with a bad tilt cylinder. The motor ran like new. I gave $75 for the motor off CL then I picked up another one with a blown power head for $50. I combined the two and with about $300 invested I had a strong running motor that was 100%. I just picked up an older (1979), 55hp as a back up simply because it was in mint condition and only $200. I have my doubts if it was ever used.

I couldn't touch a healthy four stroke for that price nor would my boat tolerate the added weight of the four stroke in that hp.

I bought an 'As-Is' 1976 15hp Johnson. It had strong compression and good spark. The seller said it came on a boat he bought and only fished freshwater and had no use for a gas motor.
I bought it a new water pump, thermostat, two new spark plugs, and a new prop. I have $800 in it and it runs like a new motor. After some cleaning and polishing it looks just as good now too.

The 40hp Mariner is most likely a Yamaha made Mariner, okay motor abit loud and thristy.
The weight is not to big an issue between the 2s and 4s(20 liters extra full would be the difference)...thats a good sized boat.
Id stick with the 2 stroke but I'm cheap and dont travel long distances to often.
If its your first boat maybe save the cash and see how often you use the boat. Later, if wanted you can upgrade the motor without losing much cash, if any.
Noise and fuel consumption would be my only reason to have a 4s...and maybe EFI but carbs have been good to me

A friend belongs to a couple local Bass fishing clubs and many of the members have some nice bass boats. They all love their 250hp four strokes....until they have a major problem and then to costly to repair. Repairs so far have been few and far between, but as time moves on, they are beginning to see a few more bigger repairs happening and folks are beginning to worry about repair costs down the road. Time will tell.....as long as we can find like new two strokes, they will still be popular.

I run what ever was cheaper right now then wait for a deal to pop up if you really want a four stroke. Sooner or later someone will dump one that won't start that needs a carb cleaning and water pump for dirt cheap.
No one knows how to do anything themselves these days and paying for labor these days soon costs more than the boat or motor did in the first place. Besides, if your going to take your family out on the water you best be able to make sure you can get them back safely. Being able to do basic repairs is part of that.

For me, its two stroke all the way. I tried four strokes and they're more trouble then their worth when it comes to working on them and most don't wear as long as a good vintage two stroke.

Motor wise I'd stick to an older model, find something clean in the 85-92 range in Johnson/Evinrude or a non-oil injected Mercury. I've got a 1972 40hp Merc 400 on a 16ft aluminum skiff that'll bring tears to your eyes at top speed. With a two blade prop and a 290lb hull its like aiming a rocket. I actually added an extra fuel tank and strap mounts to the bow to keep it level at speed. I found the 40hp as a new old stock motor about 15 years ago at an estate sale. Some old man bought it and had plans to build a wood boat but never did and the motor sat in the original crate in a closet for 40 years untouched. I'm not particularly a Mercury fan but that thing is just pure fun to open up on the river, especially after dark went no one can see what it is buzzing by in the dark.

What I've found over the years is that two strokes tend to fair better when used heavily but most smaller motors don't seem to see much use in general. A motor that can be carried tends to be hung up in the garage in the off season and not left out in the weather year after year. I find far more barely used older two strokes than I do four strokes.
Four stroke has the bigger advantage with the occasional user who may not want to or understand the importance of mixing oil properly in a two stroke but a four stroke has three disadvantages.
First is the fact that you need to keep the crankcase full of clean oil which is at the very least an annual expense.
Second is they use a rubber timing belt that will one day need to be replaced and
third they don't seem to run as long or last as long as the older two strokes.
As four strokes go, the Yamaha and Honda motors seem to be a step above the rest. Tohatsu/Mercury/Evinrude four strokes are okay but I've run into a few that were in pretty sad shape already with likely little actual use,
My original 1972 9.5hp Evinrude and my 1975 Johnson 25 and my 1979 Johnson 35 hp all run and show no real signs of wear after a lifetime of weekly use when ever the weather permits.

I've actually found it pretty rare to find one that's worn out vs killed from severe abuse over the years.
I think its got a lot to do with how most boats are run. A two stroke motor is happy running wide open all the time. Where as a four stroke motors seem to suffer when run like that. As they wear they like it less and less until you have a motor that has a ton of blow-by and is making oil every trip out dumping unburned fuel into the crank case.