I read other forums and the opinions are endless.
Here's mine:
Go road bike first. You can use it for training and race.
If you intend to go serious a tri-bike may immediately give you the feel of a triathlon but you have to careful handling this type of bike.
There are no break levers at the aerobar and the shifters at the bars are not as quick as the road bike's. I learn it the hard way. 3 months of use and I'm still not confident using tri-bike.
Aero position can be discomforting especially to the neck and line of sight of the road.
Since tri-bikes are essentially time trial bikes, it is intended for solo speed on flat ground and not for group rides or peloton and even climbing.
Price: Tri-bikes are relatively expensive than Road Bikes. Cheapest as far as I know was something like P57,000. There are road bikes that can be bought at P25,000 or may be less.
Used or brand new, i don't think it's an issue, you'll upgrade it anyway... sooner or later. What matter's most is the
fit. If the concept of bike fitting is new, google it or better yet visit a bike shop and have your fit taken. I tried at YKK (disclaimer:
wa koy labot sa YKK ha.

). Wrong fit and you'll regret it.
But if you can afford, road bike for training, tri-bike for racing. That should settle it.