well i own 2 12"ssi. So of course i say the treo. but i never heard a titanium but its a really nice sub. i know they both handle the same 750w...rms, but i put 1000w to each of my ssi and they love it. maybe the same applies to the titaniums.
My personal choice is a Treo if you have the power, the Treos were a little inefficient when i tried them. You can slam alot of power to them and they'll take it. The Ti's have been quite good for a while. I've seen a few setups with them but never really heard them. Everyone says they're quite good so it may come down to price.
Eclipse Ti can handle 1,000 watts RMS easy if you have a good enclosure. If you're going sealed, go for 1.25cu/ft. If you're going ported, 3.5cu/ft. tuned to 32Hz is the way to go. Just make sure you build the boxes up strong or you'll destroy 'em with enough power to the subs...
i own a 15" eclipse ti and i am very happy with it. i have it in a 4.5 cu ft ported box (not real sure of what it is tuned to but size and port size is to eclipse specs) and it is powered by a audiobahn a18001dt which claims 1200 rms and the sub could easily take more without much distortion. as for price look on ebay, i got mine used about 2 months ago for $225 shipped and it was in almost perfect condition. i have not heard the treo, but the ti gets extremely loud and stays very clear. look on eclipse-web.com for specs (the sw9152) and check on http://www.carreview.com/cat/car-audio/subwoofers/eclipse-fujitsu-ten/PRD_49323_ 1808crx.aspx for some reviews on the eclipse.