What's a good setup for an experienced but rusty cyclist training for his first triathlon? I've got my first (sprint) triathlon coming up at the end of July. I need a bike to train and race on. I'm comfortable on both road and MTB style bikes, but haven't ridden on either in several years. I could see myself using either style well after the tri is over. It doesn't seem like I should spring right into a tri bike since that's a pretty specific setup and I wasn't too comfortable on the one I test rode. I've also seen a tri-cross bike that looked like a good combination of a road and off-road setups, though I haven't had a chance to test ride one yet. My immediate goal is just to finish this upcoming race, but I would like to continue biking more afterwards and that may or may not include future triathlons. I've got a budget big enough to put me on an entry level tri or a mid-level road bike. Or should I just go with a generic road bike and be able to get an entry level MTB bike too?

I'm not necessarily looking for a specific make/model (unless there's a hybrid you think I should try), more for what would be the best most practical configuration given my needs.
You probably don't want to spring for a specific time trial bike straight out of the gate - you might hate the triathlon experience, for one thing! Given that you're planning on using the bike after this events, albeit possibly with other events down the pipe, I'd be looking for something that you can get the best use of after the event. 

So a road bike would seem to be the better bet - possible with a set of clip-on tri-bars; while not everyone gets on with them (not all road bike/ride combinations work with the correct geometry) they are generally considered faster - you're in definitely in a drag-reducing pose and they force you to stay still a lot more, which is better, too. 

A tri-cross sounds, erm, intriguing. A time trial/triathlon bike will be set up to encourage a mono-position with speed, probably with a longer, lower back/body position. A cross bike generally has more clearance and encourages a head position almost over the bars, a shorter top tube and more clearance to enable better control on a uncertain terrain - these would seem to conflict ... 

Over the years I've seen triathlons done on many types of bikes - even sit-up-and-beg shopping bikes with a basket! You won't be doing it for speed, but you'll finish. 

Use the bike that you would think you'll get use off after the event. This will result in several things: 


you'll have spent your money on something you'll use 
you'll finish the triathlon 
you'll have set yourself a (beatable) target time for the next event 
you're going to be in a much better position in a few months, after the event and after a bit more training, to know what'll work for you. 


Then get all of the above: once you start racing, commuting, shopping and training on the bike you'll be able to justify a time trial, commuter, road and MTB ... and even then you'll probably find another type you're missing!