If you want quality, youre probably better off with Blitz. This isnt to say I dont like AMOS, I actually do, it's a lot of fun, and if you chose your game style/game play carefully it can still produce results better than its reputation (check out Babeanoid for example).
They are quite different though. Blitz can be completely system friendly if a person choses for example (although it does have its own "blitz mode" that hits the hardware more directly). Using it in a system friendly way gives you access to all sorts of nice things like opengl, sdl, mui,reaction,etc.,etc. Basically if you can do it with C/C++ you can do it with blitz, and honestly, unless you use older amiga specific compilers, often faster. Blitz can also use inline assembly directly (I think AMOS can to, but I dont recall it being as "Seemless" as with Blitz).
One observation Ive noticed with AMOS that isnt often mentioned though, is that it seems to scale very well to the hardware its running on (ie. games will often perform proportionally to the hardware its runing on). Naturally most code regardless of language will be faster on a quicker system, but for whatever reason it really stands out with AMOS. Having said this though, Blitz will be faster 9 times out of 10 anyway.
Personally I'd suggest it comes down to your plans. If you just want to knock out a few games to have some fun with no plans to make bigger projects down the track then AMOS is probably the way to go, but if you want to attempt some "serious" projects then in humble opinion, Blitz is actually one of the best languages to make classic amiga games with.
edit. Ah, I just thought I should point out that you dont really have full access to AGA with AMOS.
Technically its an OCS/ECS product, but there are (unofficial/undocumented) ways to take partial advantage of AGA with AMOS. For example you can still ony use up to 32 colors, or ehb, or ham6, plus youre limited to 16 strengths stil per rgb channels, but you can have 64 color or ham6 640x512 screens. You can also use double bandwidth (AGA) modes. Again this isnt officially, so you'll have to hunt around aminet for all these sorts of tidbits and examples. There's an archive I found called "allplaton" that's a good source for these sorts of tibits (although it might be an ida to get your head around AMOS itself before you start using these things
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