The idea of a turbo timer is that it keep the engine running for a pre-set time after the key has been removed - this for the purpose of allowing the oil to continue circulating and cooling down to reduce the heat in the turbo and related. Supposedly. I've read that this is mainly a carry-over from the days when turbos were not water cooled like they are today, and that they're really not necessary. Personally, I'd just sit in my car and let it idle for a minute or two before I shut it off and get out if I'm really that worried about it
If you really want one, they all basically do the same thing IMO. The main differences are the "bonus features" like different timer modes, or also having a tach and speedo and voltmeter or w/e built in. They're all around the same price ish too, so if I were going to pick one, I'd just pick whichever one I liked the features on.