The only reason I'd go with Tekkit Classic is for IC2 and EE2, the latter tending to devour all of your everything, time included, sooo not totally recommended. I think it also has forestry? I can't remember if that was still removed.
The new Tekkit main replaces IC2 pretty well though with Thermal Expansion. Also has Applied Energistics and Dimensional Doors, both of which are godsends. Only thing I miss really is forestry stuff, solars, and very rarely transmutations from EE2, but everything there is nice, it's a decently updated version of Minecraft, and it runs well even on lower end systems (that can handle Minecraft's awful code to begin with).
Tekkit Lite I've not touched, really. Don't see much of a point to.
Another alternative I've yet to try too much of is Feed The Beast, buuuut really it has a stupid amount of mods in it.
Anyway, question for all. If I bothered to upgrade + install plugins and whatnot, how many people would join and at least occasionally play on a Blocklander-hosted Tekkit server?