Which bike?

Do you have a bike?  If so, that's the bike to ride for your race.  If not, spend a few hours considering what you'd use a bike for besides the triathlon.  I did my first 2 triathlons on my steel frame mountain bike (it's what I had) and had a ball.  I then bought an old road bike that I rode for the 2006 season to do my half-ironman, a vintage tri-bike, and finally, a new lower end road bike (it listed for $1600 and I got it for $600).  The tri-bike was $500 (in 2006, it's a 199X model) and the fitting and tune up another couple hundred.  I've ridden it for 3 Iron distance races, so I feel I got my money's worth out of it.  If you don't currently have a bike, consider a "flat bar" or fitness bike.  They're slightly heavier than road bikes but with a more upright riding position and slightly wider tires (tyres).  Around here, they can be had for maybe half the price of an entry level road bike. 

Repeat after me:  "There's no such thing as a cheap bike." 

Step 1:  Determine approximately what size bike you should ride.  There are a few online calculators. 
If any of these are that size, you can investigate further, but understand that even relatively small sounding differences (1-2 cm) can make a significant difference in how comfortable you are. 

Step 2:  Visit your local bike shop and price the following:
Fresh tires and tubes
Fresh brake pads
A basic tune up (Nothing replaced) and inspection
Truing wheels
A pump, tire changing kit, and seat bag. 
Any additional accessories like water bottle cages that aren't already on those bikes. 
You'll also want a helmet and glasses (prescription or otherwise) for riding. 
(Now you see why I say there's no such thing as a "cheap road bike.")
Most of the routine stuff you can probably do yourself.  Truing wheels and adjusting the derailleurs you should leave to someone who knows what they're doing, plus the shop mechanic knows what to look for regarding frame damage. 

Step 3:  Consider if you want specific bike shoes and clipless pedals or will use platform pedals. Shoes and clipless pedals are likely to set you back twice as much as those bikes you mentioned. 

Then you'll start looking at bike shorts, triathlon shorts, gloves, etc. 

The skinny, high pressure tires make riding a road bike much more efficient, but the riding position takes a while to get accustomed to.  Some people never really like the "crouch" position when riding with automotive traffic.  I bought a road bike in 1981 and rode it back and forth to work at summer jobs. I got the mountain bike around 1998 because most of my riding then was around the neighborhood with my children and those skinny, high pressure tires and crouched over position aren't so good with glass, gravel, and storm grates.  The wide, low pressure mountain bike tires worked much better for that riding and I wasn't constantly patching tubes.

True story:  I'd just gotten the tri-bike and had met up with a friend for an hour ride.  For whatever reason, I had to return to the car after a couple minutes, so I told him to keep riding and I'd catch back up with him. So...I'm humping along trying to catch up with my friend, probably 18 mph (30 kph) and a guy riding a MOUNTAIN BIKE cruises past me like I'm standing still...

Stick around, ask more questions, and let us know how your race goes!