You're also kind of ignoring that 16:10 is also very prevalent in modern day monitors, of course. Although given the choice between 16:10 and 16:9, I'd go with 16:9 (the wider aspect ratio).
You guys are forgetting about the best game creation tool out right now: Stencyl. Seriously, if you haven't, check it out.
I think it shouldnt matter about limitations... it all comes down to what you prefer working with... If you are new, I suggest giving a test run with hacking, and/or some fangaming programs also... see what your preference is. Take a look at hackers. They have more roadblocks to work around than any fangaming software out there, due to the limitations of the platforms they are based on. Software like Game Maker, and MMF2 don't have those types of limitations... They may have their own ways of working which can make some aspects difficult, but it can all be worked round with experience and know how... arguing over which one is superior seems like a moot point to me. But, thats only my opinion. BTW I use Game Maker... I agree that there are many unlikeable things about it... but as I said, they can be worked around... Besides, the reason its less prominent as a popular fan game program is because kids are the majority who use it... Do a google search for a game creation program and its the first thing to pop up. Anyone don't believe me, ask this guy screen-named Mercury... maybe you've heard of him...
Im highly considering rereleasing Sonic Classic 1 in 16:9 ratio. I also am considering making Sonic Classic 2 16:9 only.
I've been trying to learn Construct lately and probably one of the biggest issues is the lack of tutorials compared to something like MMF2. I know that Construct is definitely capable of what I want to do but I wonder if I should consider MMF2 for the sake of having more resources. Considering how so few projects use Construct, it would be cool if I released a fangame using that program since it's a pretty underrated program that's quite good. As for Game Maker, it has never really interested me. It's capable of good things, but I generally don't care for how it's setup for playing games and the fact a lot of people can't even run Game Maker games makes me never want to use it. When I play Classic 1, since the full screen mode is in a resolution that's smaller than mine which results in the game looking blurry, I maximize the window so the pixels are sharp and then use my monitor's 4:3 mode so it isn't stretched. Definitely not ideal since it would run better at a normal full screen mode. I know that in VVVVVV, for full screen resolution options, there's an "auto" setting for full screen that allows it run at an ideal resolution. Perhaps you could implement something like that?