There are too many variables to answer that question. It all depends upon what you want from your shave, and your beard and face type. For everyone who swears by the 23C, there is another who loves the 34C, and another who loves the 38C and another who loves the 11C.
They are all great tools, and each has its followers and devotees, but only YOU can decide which works best for YOU.
Or maybe you could at least give us some more details?
Didn't use the 11C but I prefer the 25C over the 15C; but the head is identical.
Best Merkur open comb head however in my collection is not on a Merkur but on the new Mühle R41(model 2009).
Far better fit and finish and a tad more aggressive (but still very mild) for just a few bucks more....