It depends on usage:
If your only going to use either of them for in-game battles then I would recommend Sawsbuck for his decent attack and speed and also his okay defenses. Plus if your concerned about HP then you'll be happy to know that Sawsbuck's HP is slightly better then Whimsicott's. However, if you're going to use either of them for competitive battles then I would recommend Whimsicott. Although Sawsbuck would be great in a Sunny Day team with his Chlorophyll Ability, Whimsicott has a unique trait with Prankster. With Prankster as its ability that means non-damaging moves go first (such as Leech Seed, Cotton Guard, etc...) and would be perfect for competitive battles!
Here are some movesets I would use:
Sawsbuck (In-Game) Sap Sipper
Double-Edge(with horn leech you can regain some health)
Whimsicott (Competitive) Prankster @leftovers
This is a bit tricky. You could use Grass Whistle on the first turn and put the opposing pokemon to sleep then proceed to leech seed and cotton guard. To keep with the regaining of health theme you could opt for Giga Drain which would gain a STAB and would use his okay sp. atk or you could use U-turn which would be useful to get out of a situation like facing a special sweeper (because Whimsicott's sp. def is lacking) while still causing damage. The only down side is Whimsicott's atk stat is also lacking and he wouldn't gain a STAB bonus.
Personally I like them both.