I have the 27 and it's a dandy gun. If you want to make it more comfortable for target shooting (not that it's not comfortable in stock form), just use longer mags for targets. Another nice thing about Glocks. Mags can be had easily and are dirt cheap compared to most others. I have five mags for my 27 in various capacities. If I had that many for my S&W M&P 45C, I'd have more money in mags than I do in the gun (but that's another topic).
Like you I own a P229 and added a G23 due to it's lighter weight. Mid-sized guns are my preference, they just suit my tastes. I carry the G23 nearly everyday in a Dale Fricke Archangel. The Sig or a 1911 still gets into the mix every once in awhile.
my feeling is, if you can shoot the 27 well with the standard magazine in place, get it.
if you feel the need to have an extended pinky finger support on the 27, get the 23 instead.