I own both, I've taken 2-day training classes with both, but I usually only pack the 27 as a back-up to my G23 or G35. In light clothing, I usually pack the 26 as primary, with a J-frame Smith as back-up. While neither are bad/un-fun to shoot, the G26 is a little bit easier to shoot than the G27.
If you can get the "Law-Enforcement" Glock OEM magazines legally in your jurisdiction, the G26 has a bit of an edge in that it will hold 12+1 9mm's, while the G27 only gets 11+1 .40S&W's. BTW, all these mags consist of is a standard "sub-compact" mag body with a Glock +2 extension added. The Pearce PG39 extension gives the same capacity increase as the Glock unit.
Since you already own a G17C, if you get the 26, you won't have to stock a new caliber, plus your existing magazines will be compatible as spares.