As a newbie tri guy myself....... It would be ideal to have both, if possible. If not possible, you need to decide the MAJORITY of riding you will be doing. If you are strictly tri/time trial, then the choice is obvious. I would caution doing "tri set-up" on a road bike (I am sure others will disagree). The geometry is completely different, and in my opinion would be counter productive due to the decreased torso-femur angle. Of course continued trial and error of positioning could remedy this, but a BIG PAIN nontheless.
I purchased both and wound up spending 90% of my time on the tribike. I use the roadbike forlong group rides, and when my tt is in the shop. I have considered selling my roadbike as I can get away with using only my tt.
I use the tt only because i am so much more comfortable on it, so that has a lot to do with which bike i choose to ride most.
I had similar budget and purchased a Felt F75 road bike. It has been a terrific bike for getting into riding and I have done 5 races on it. I did start to covet tri-bikes after my first race, and finally picked one up this month. My tri-bike is sooo much more comfortable, and noticeably easier to run off the bike than my roadie, so I am thrilled to have purchased. I still think a road bike is the best choice to get into the sport, but don't be surprised if you find yourself back in the bike shop looking at those sexy aero frames.