Ti is nice and light and expensive if you got the cash go for it.
Aluminum is stiff, and depending on how you ride the avergae life span of aluminum is 5 years. if you break or bend it, you gotta chuck it as it is unrepairable.
but I always buy steel, and I have riden the ti and aluminum quite frequently. if you have alot of money to buy TI then you may have to opt for lesser components. if you get steel (a little heavier) you cabn use that left over cash to get the top component and gear. what matters is not so much the frame but components that work and last.