While the alloy one is a lot cheaper it is also a lot harder to look after than the titanium one.
With winter approaching and the pending road salt gritting soon to start, even if you do not intend to ride during the winter months, there will be times when its just nice to have a whizz out isnt it!
Alloys + rock salt = white fur ball the next morning!
IMHO spend that little bit extra and go for the Ti one, you know it makes sense?
Got to agree with the others... this is what happens if you do not keep an aluminium can clean in winter. The extension is stainless and polishes up lovely but the pitting will never come out of the aluminium.