The regular forms both have the same stats total of 600 with 100 in each stat. Shaymin's Sky form loses a bit of defense for a good increase in Speed and Special Attack. Mew's 100 stats does mean it can me moulded to any role e.g. wall, sweeper, tank.
Shaymin's Sky form has the Serene Grace ability which is pretty great. Mew has Synchronize which isn't too bad.
The movesets is where Mew really shines, as it can learn every TM, HM and move tutor move, making for incredible and unpredictable movesets. This alone pretty much wins it for Mew IMO.
If you are able to get Shaymin in Sky form, give it a try as it makes a good Special sweeper. Otherwise, Mew is by far the best.