I’ve got a late (V reg) T’bird Sport and LOVE it! Abso bloody lootly LOVE IT!
In many ways it’s the most “accessible” of the T300s in the sense that, although it is, as far as I know, fundamentally the same large diameter spine frame; welded on rear monoshock subframe and the T3 series motor, the machine feels - subjectively - more compact, lighter and lower than the rest of my motley collection of ‘clunkers’. The seat height is definitely and noticeably lower than the ‘standard’ Trident/Trophy/Speed Triple etc. variants of the T300 but I have NO idea how they manage that as the wheel sizes do not appear to be vastly different to the cast wheels of the other T3 range bikes. It’s probably a combination of a slightly more sculptured seat foam and base and, perhaps, slightly lower rear subframe …. but that’s personal conjecture.
As much as I love ALL my T3s, I can’t deny that the TBS is/was the ‘go to’ when I’ve got more than one on the road - didn’t happen AT ALL last year as a result of ‘other’ biking commitments - and it’s largely down to the ‘manageability’ of the machine - wider, tubular bars, slightly lower pegs(?) and sculpted seat all make it seem easier and safer to manhandle. Bear in mind I’m shrinking in height from an optimistic 5’5” with an even more optimistic 29” inside leg measurement (and you can stuff off if you ‘re expecting that in metric!!) I can get the soles of both feet down though don’t think I could quite flatfoot it. It also - again subjectively - feels much lighter than the Tridents, Speeds, etc., though I’d find it hard to rationalise or explain that subjective feeling.
You do have to bear in mind that these bikes have been out of production for more than 20 years now (I think the Thunderbirds were produced up until 2004 but the T5 series was introduced in about 1998/9 wasn’t it?) so genuine, factory spares availability is abysmal and many of us swim in the shark infested waters of a limited stock of secondhand or NOS spares/replacements supplies. The Bay of all Evil may not be your friend but it will be a frequent companion on your hunting forays for spares. Sprint Manufacturing is good for some stuff, including carburation spares, but genuine factory stuff is the proverbial rocking horse poo.
As far as I know the three into two stacked or three into two individual exhaust/silencers arrangement was purely styling (bit of the Craig VETTER type styling to the two ‘stacked’ silencers perhaps?) so the only viable advantage of one against the other might be the argument for accessibility to the left hand side of the rear for the stacked system bikes. No big deal and my personal preference for symmetry is definitely better served by the later style.
As a T300 variant the engine specs are broadly similar across the range so I THINK valve clearances checks/adjustments are the same (12,000 miles?) but the T’bird; Legend etc., has a lower horsepower output so I think it’s probably not quite as critical as it might be on, say, a Speed Triple or Daytona. Cost? Not sure but I’d guess on budgeting for 3 to 4 hours labour and a few replacement shims? That’s an uneducated guess, though so worth checking with your local, go-to mechanic. Sorry, Technician.
They are really good bikes to own and ride and the Sport variant seems to be the best equipped of them all - it’s also (albeit subjectively) the best looking …. especially if you like the symmetry of two disks; two silencers; two clocks, etc. 
Good luck with your search for one - get the right one and the rewards will be as handsome as the bike itself.
