B767s

For:
Larger, more spacious cabin, less congestion in the aisles
More room for storing carry-on
No middle seats
Greater chance of a CSD, so service may be more professional/efficient
A chance of a switch to a long-haul 767 (some switches are already scheduled, so check seat maps for your travel dates)
More loos (3 for a club cabin >9 rows)

Against:
Tatty interior, high probability of arms falling off, tray tables falling apart
Poor picture and sound quality, some IFE monitors will be broken
Tiny IFE monitors in bulkhead rows
Risk of call-bell-itis (non-stop pinging of the call bells)
Risk of technical fault/delay

A320

For:
None really, except you avoid all the problems of the 767

Against:
Middle E seats in use, so expect some grumpy passengers on board
Less room to store carry-on
Possibility of non-IFE equipped plane (the plane should have IFE, but there is a small chance they will switch to a non-IFE equipped plane)