Originally Posted by SethAZ

I'd stop the hand wringing and just keep the 105 and get on with life. I personally see no point in this day in sticking with a 9-speed group at this point in cycling history, where 11 speed has been pushed well down into the budget group territory.

11 speed is just nicer. While 9 speed obviously works, and once was the new thing itself, the 1-tooth shifts that make it much easier to keep a consistent cadence under inconsistent conditions tend to be clustered at the higher end of the gear range, and on a 9 speed make it less likely that you're getting close shifts in the speed range you're typically at.

Plus you don't want to psyche yourself out. You say this is for a secondary bike for specific purposes. What are you telling yourself if you cheap out and downgrade from an already cheap gruppo (105) to a dirt cheap gruppo? If you act and think like this bike isn't important, how motivated are you really going to be to go out and ride it under those scenarios to which you've relegated it?