First of all, the current bases for each type are as follows;
B737NG -
Newcastle, Luton, Belfast
Liverpool, Edinburgh and Glasgow are being converted to Airbus by the end of the year
AIRBUS -
Gatwick, Bristol, Stansted, East Midlands
easyJet are pretty flexible on basing. They will offer you a base on successful completion of interview, and you have the right to apply for a base transfer. Obviously, if you are a Captain these transfers are limited as I know the main command bases are currently Luton and Belfast. If you were to be offered a command, it will most likely be at one of those 2 bases initially.
I could not tell you whether there would be a greater command chance on the Boeing or Airbus, however there are going to be more Airbus than Boeing, so it could be either way.
Hope this helps
LVLCHG