I bought a quite OK M2 but with dirty viewfinder and I preffered the IIIa by far, even for focusing (with the SBOOI)! So I ask a refund for that M2. Plus the difference in size is gigantic... Those Barnacks are cutely tiny. Anyway, I am waiting for a mint M4 that should show up in a few days, so maybe I'll change my mind! The price difference is something but may vary greatly from where you buy it and the amount of time you search for THE good deal.
The IIIa should not be reguarded as a cheaper leica counterpart, because its the last leica designed when Barnack was still alive, and that the more advance model are not as beautiful (like the single frame body cast that was made to reduce cost, even If it makes it more 'sturdy' to some), especially the plastic involved in the shutter advance knob is not that terrific! That's my humble opinion. Anyway later Barnacks can be smooth because of the bearing ball mecanism that was introduced in the IIIc.