Yerzymey and gwEm put it well. STE is optimum and leaves no room for problems: you can upgrade RAM cheaply, have audio & TV out and you can use up to 3xYM + 2xDMA channels as said before. If you are interested in sample-based tracking (like Protracker or Octalyser modules) STE is strongly recommended. Though, you can do that also on normal STF, e.g. Digicomposer and specially Protracker ST are pretty powerful - the only difference to STE is the sound quality.
Alternatively, if you want to stay low cost and go for pure Atari YM-chipmusic only, a pure STF model is a good choice. Several trackers like Insignia/Trisound, MusicMon and maxYMiser support digidrums (aka samples in the nonatari world;) within the 3 channels.
For STF, you will need a monitor connector though if you want to use color screen. In EU, SCART-Adaptors are common. If you have the chance to get a STFM you should prefer that, as it has TV-modulator inclusive and costs as little as STF usually.
Speaking of the models, I realised that most STF models tend to yellow quite a bit, while the newer STFM is staying in bright shape (the eye tracks before the ear!). Might be due to different plastic factories in different years. So much to my theory about this
Yerzymey: Did you take a look at DBE-Tracker from Sector One? It is pretty amazing on STE, software mixing up to 32 voices if I remember correctly - you can guess the replay quality though. Anyway, impressive replayer. Also good for checking som PC formats like basic S3M/XMs on STE.