Hard to imagine XPR will be anything more than a novelty. The secret to BitCoin are two fold, there is no central control and there aren't so many outstanding.
Once the source code is released and new servers start popping up all over the place Ripple will be as distributed as Bitcoin. And since it is quicker and can be used to make convenient fiat payments it could see much faster adoption. If it is true that OpenCoin intends to stabilise the value of the XRP, this too will help its adoption.
As far the number of coins, XRP too has a fixed, finite supply, just like BTC. And, again as with BTC, not all of them are in circulation. BTC issues them as a reward for mining, XRP is premined and given away to new users.