Ranger primary:
- slightly better armor with good elemental resistance
- better weapon damage, beefed up with Expertise skill
Mesmer primary:
- slightly more base energy, most armor adds even more
- Fast Casting skill for whipping those counters out in time
There are differences in armour and energy recharge rate.
Also you don't get access to Fast Casting if you're a R/Me and don't get access to Expertise if you're a Me/R.