This is going to be a straight up assessment between the old 10-speed and new(ish) 11-speed Shimano 105 drivetrains… with a little Ultegra mixed in just for fun.
First, if you don’t like reading about bike stuff and you currently have a 105 or Ultegra 10-speed drivetrain, I’ll save you the trouble. Finish this paragraph and be on your way. Stop everything else, immediately. Either hibernate your computer and head over to your local bike shop and have one of the employees order an Ultegra or 105 11-speed drivetrain or, if you can install the set yourself, order one online. Just remember not to take the stuff you bought online to the bike shop. That said, upgrading will be worth every penny if you can reasonably afford those pennies – and there will be a lot of them involved.
If you like to read about bike stuff, let’s continue with the always important why.
Shimano’s 10-speed drivetrains are famously flawed. Or perhaps that’s infamously? How about notoriously?. The simple reality is, the springs in the rear derailleur are reportedly, and quite obnoxiously, too weak so the shifting, when viewed against the 11-speed drivetrain, is suboptimal once the springs start to… erm… get sprung (stretch over time). This isn’t to say the 10-speed is crap, because it isn’t. It’s just not as good as the 11-speed because they worked out the spring tension issue in the eleven speed edition.
How this flaw manifests itself in the 10-speed system is that, once a rear derailleur is “of a certain age”, it becomes increasingly difficult to keep enough friction out of the system enough for the weak spring to allow the drivetrain to shift properly all the way up and down the cassette. You can install brand new cable housings, new end caps, stainless steel cables, new cable liner (at the entrance and exit points of the housing/frame interface points). You can literally do everything right and the derailleur won’t dial in unless the barrel adjuster is dialed in perfectly, within a quarter-turn.
Eventually, that quarter-turn won’t be enough and you’ll need a new derailleur.
Now, for the time being, you can still pick up a 10-speed 105 derailleur but how long this will last is anyone’s guess. You can also pick up a refurbishing kit, something I was supposed to try over the winter but never got around to, that comes with a new spring. I’ve heard refurbishing the rear mech helps considerably.
As I alluded to earlier, this flaw was rectified in the 11-speed drivetrain. My wife has 105 11-speed and I have the 10 on both my Venge and Trek and I can tell you unequivocally, the 11-speed is vastly easier to keep operating smoothly. It’s not a night and day difference, but it’s big enough to notice. Especially when that tensioning spring starts to weaken after 20,000-ish miles.
To wrap this up, go back to the top of the post… once your 10-speed drivetrain starts to wear out, I’d make the jump to 11-speed. It’s worth the headache savings alone.