Seen them both, own S6. I still love S6, and it has a great soundtrack and really good editing.
They're both very good, and both very different.
S6 is all hand-rails with park and powder booters. Lots of good, young riders.
TBC is mostly pow jumps and amazing gaps in the BC. The jumps are much bigger in TBC, but there is almost no hand rails and there are a couple of segments I'd skip, the soundtrack is also a little worse than S6. TBC also has older riders, and more big names, but it doesn't always mean better skiers.