I have had 4 Titanium Frames (still have 2) The first one I sold to friend in a moment of weakness, (it still looks & rides like new at 20+ years old) the second one I bought of
ebay it was super light with double butted tubes, this one cracked but not at a weld, it cracked at the butting transition (very thin tubing) as it was second hand the frame builder couldn't offer me a replacement frame but did offer a generous discount off a new one if I returned the cracked frame to him so he could investigate the failure.
As I was familiar with working with titanium and knowing what a super metal it was (the reason I bought Ti) I decided I could repair the crack, a friend a t work was the best Ti welder I know so he welded it for me, we had all the facilities for weld examination and the weld passed with flying colours, in the meantime I bought a carbon frame and put all the bits from the Ti on it, so I built the welded up to run fixed & ran it for 2 years before selling it, I think its back on gears now & still running.
The carbon was a decent bike, but the ride with exactly the same components that were on the Ti wasn't as good and just as heavy, so it had to go, the 2Ti I have now are a dream to ride both very light, a lot easier to keep looking good than a painted frame, (why add the weight of paint & lacquer when its not needed) I also have 2 aluminium frames these are decent frames but again not as forgiving as Ti,
The Spa Ti looks a comfortable frame & if you like it I would say get it as long as you are happy with the ride, Ti frames can be made stiff but I don't think Spa is one of them.
The bottom bracket & head tube are just tubes because that's all you need, IMO they look a lot better than the over sized ones on some carbon frames.
Aluminium is not corrosion proof, subject a bare aluminium tube & a bare Ti tube to the elements salt spray etc for a year & see which one survives,
Aluminium seat-posts corrode & expand to stick in the frame, titanium picks up on titanium and if this happens you have a big problem, I have had titanium collars pick up on titanium shafts on several occasions, the trick for both Aluminium & Ti is cleanliness & the right anti scuffing paste, I only use carbon posts in my Ti frames.
Of course the down side is the expense, but in my opinion worth it.