I have been using the ST and Amiga for many years despite my relatively young age.
I have had most Amiga models, the A500, the A500+, the A600, the A1200, A1200T, CDTV...
However I have only ever used the Atari 520STFM.
I think unlike the Amiga, it is quite easy to get by with 512k on the ST. Most games from the ST's heyday will work fine on a 520 model.
I would recommend the 520STFM on that basis, it has everything you need, TV out, monitor port etc.
If you need to run 1Meg games then consider the 1040STFM though I have never used one of these I am sure they are good machines.
On the Amiga side of things, I have had several A1200s, and whilst they are nice machines, I would not recommend them for retro gaming.
They have compatibility issues with a lot of older games. Most games pre-1992 will have problems with an A1200.
The original A500 only has 512k memory and the graphics chip is inferior to later models, so I would avoid this model.
The Amiga 500+ has battery leaking issues and not quite as severe compatibility issues as the A1200.
I think for classic gaming your best bet is either then Amiga 600 or a 1Mb Amiga 500.
For 100% compatibility go for the 500. For a slightly better chipset and a smaller case look at the 600.
Hope this helps :-)
Reptile