I have owned both for years now. I owned the old school 300rms/1000peak type-rs (3 of them sealed in a mustang gt). The new 500rms were a huge improvement. After Alpine put out the type-x I waited till they had some of the bugs worked out and bought one 12. Compared to the new type-rs it was still a little louder. I personally LOVE the really really low lows, the deeper the better, and it did get much louder at the lower freqs than the rs.
"-if you're after more low-end, then switch to the x's, they have an almost unnatural peak at the extreme low frequencies" - I agree 100%
For the $ shipped off
ebay 2 type-rs are within $10 of 1 type-x. Giving 2 subs 1600w, or one 1100, yes they would be louder (assuming they would be wired at 1 ohm and 2 respectively to your hifonics amp).
One type-x compared to one type-R you will see better SQ and SPL with the X.
2 Rs vs 1 X, get the Rs.
One thing to consider is buying one X, then waiting till you have the cash to buy a second sub and amp.
I built the ported box exactly to alpine specs, and it sounds great. I have had numerous people argue with me that I should have bought 2 subs instead of one, but I wanted the cargo space. Then they actually sat in my car and said things like "Thats just one?!" and "****".
To give you an idea I got metered and hit 142.8db with the one type-x with a 38-48hz sweep, and I have it set up for SQ. You could be shocked at how good the X is, but for your money and given the choice of 2 Rs vs. 1 X, I would just get the Rs. Thats just my $.02.