Well, it depends on several parameters..
1- You should make sure that the 21 channels can support a BCCH plan pattern looser than 4X3
2- Mixing the BCCH and TCH channels in adjacent way will expose them to adjacent channel interference. While if you isolate them you will not have this problem, besides a small interference with 4X3 pattern.
3- remmember that the number of TCHs should be more than 2 times the number of TRXs in a cell to get good frequency hopping gain.