What kind of bike to rent?

well, i don't have any experience renting bikes, but i'll tell you what i would do if it were my first time. the general principle you will hear often is to "race how you train", or equivalently, "change nothing on race day". with this goal in mind you should rent a bike that is as close to your mtn bike + slicks as possible.

So the first thing to do is call the rental shop and ask if they'll put slicks on a mtn bike for you. While you have them on the phone ask if they'll throw your pedals on. If they say no to the pedals, take yours and a wrench, and swap them out yourself. If there is no chance for slicks, then rent a touring bike that will keep you upright in a similar position over handlebars (no drops).

Most importantly, good luck and have fun!!

that is what i would do - ymmv

I'd normally agree with the "nothing new on race day" saying, but in this case, you'd be giving up a good amount of speed by choosing a mountain bike (even with slicks) over a road bike. Most likely, you'll have the chance to ride it for at least a day or two before the race, so I'd say go with the road bike.

Prior to buying my road bike last year, I was considering using my mountain bike with slicks in my first triathlon. After test-riding a road bike, however, there was absolutely no comparison.

My first road bike ride was the night before a Tri when I had picked up the rental.

I rode it for about 20 minutes to get used to shifting and braking and that was it.

I missed a lot of shifts in the race but not once did I wish I was back on my MTB w/slicks.

The weight difference and the perceived speed is night and day between the two.

I highly suggest renting the road bike.

Definitely bring your pedals, it takes two minutes to change out so why not.

Nothing new on race day will not apply as this bike (either one) will be different. Get the road bike or even better if you are a good rider, go tri. The seat will be new and as everyone knows, that is the hardest part of new bikes, other than the total change in your case of geometry and position!

well - there are shades of grey to "nothing new on race day". so the goal IMO is to minimize the change. I would expect a road bike to put more strain on your neck and hands then a bike with more upright seating. a road or tri bike is definitely faster if you are comfortable enough with it. just be aware of the possible differences.

I'm fairly new to the bike in general, and did my first race on a MTB (knobby tires and all). I finally bit the bullet and bought a road bike. My first couple of rides were awful. The handling was different (magnified by my inexperience), my lower back was on fire from the new position and my hands/arms hurt from trying to get out to the brakes/shifters. I can't imagine going on my first (or second or third) ride and then having to transition into the run.

I felt as though I had pulled my lower back muscles for a good week and a half after I started on my new bike. Granted I have virtually no core muscles, and I understand that you are supposed to use your core to support you, but unless you truly have abs of steel, I think the new position takes some getting use to. Obviously that is just my experience but I wouldn't want you to have to go through it in your first race. I suggest renting the MTB and trying to get the slicks and pedals put on.

Best of luck in your race!