New Ultegra is 11 speed, and AFAIK 11 speed cassettes don't fit on 10 speed hubs*, so you would have to either have your back wheel rebuilt with an 11 speed compatible hub or replace it entirely with an 11 speed wheel.
As for groupsets
RIBBLE let you customise your options, so you can pick crank length, compact or double, cassette size etc, etc.
Personally I'd stick to your Tiagra and save the cash, but if you want to 'upgrade'** then I'd stick to 105 so you avoid the whole 11 speed issues.
*from Velo News;
Forcing 11-speed on 10-speed hubs
Dear Lennard,
I’ve just ordered an Ultegra 6800 group to build up a new bike. The group I ordered does not include hubs, as I intended to use wheels I already have that use Ultegra 6700 hubs. But now I’m seeing information (or mis-information) that the 11-speed cassette that comes with the Ultegra 6800 group won’t work with the Ultegra 6700 hubs. So, what’s the real story here? If the 11-speed cassette won’t work on a “stock” 6700 hub, is there any workaround, like replacing the freehub?
—Tom
Dear Tom,
Indeed, that information is correct. The Shimano (and SRAM) 11-speed freehub is wider than the 10-speed one, because the 11-speed Shimano (and SRAM) cogset stack height is taller than 10-speed. Even if you were to get the cassette lockring engaged by shoving those 11-speed cogs together as tightly as you could while you were screwing in the lockring, you wouldn’t get much thread engagement, and the first cog’s splines would not overlap the 10-speed freehub body’s splines. I’ve tried this; the lockring will thread in maybe one turn, but the first cog is not engaged; it will spin until the lockring binds it down as it turns it together with it. But if you were to apply a chain load, that cog would either just spin or it would continue to force the lockring to screw in more until the one or two threads holding it stripped, and then all of your cogs would come loose.
And even if you were to feel comfortable with the thread and spline engagement you got, the face of your lockring would rub on the inner face of your dropout when you installed the wheel in your bike. Yes, you could throw a little washer in there between the end of the hub and the dropout, but you’d still have the problem of a disengaged first cog and a lockring only holding on by the hair of its chinny chin chin.
The fix on some wheels is to get an 11-speed freehub body and a different set of axle end caps or spacers or a new 11-speed axle, but not all generations within a given brand are interchangeable. The fix is to interchange the freehub bodies and put more spacing on the axle’s drive end and less spacing on the axle’s non-drive end. Then you’d re-dish the wheel more to the drive side. I recently went through this with Zipp, and the re-dish amount was 1.85mm. Thing is, only 2012 and later Zipp wheels can be converted to 11-speed.
However, there is a fix for those with wheels that are not convertible to Shimano 11-speed freehubs but that offer Campagnolo freehubs (as you might imagine, Shimano is not one of those). You can install a Campagnolo freehub body on your wheel and get a Campagnolo 11-speed cogset. It will shift fine with a Shimano or SRAM 11-speed system.
And in case any of you were wondering, if you have a 10-speed Shimano or SRAM system and buy a wheelset now, you can still run 10-speed cogs on that wheel. To put a 10-speed cogset on an 11-speed freehub, you just put a spacer behind the largest cog. Many 11-speed wheels come with this spacer.
―Lennard
Read more at http://velonews.competitor.com/2013...or-10-and-11-speed_301392#HdHalgSZI0TGY7jD.99
**IME you won't notice any physical improvement in shifting with 105 over Tiagra. Your cables will be neater as they'll be under the bar tape, but there's even an argument that under tape cabling reduces shifting performance as well.
