I've taken both because I get whatever the insurance company has listed in their formulary -
Personally I've found that they both work at the same speed (lithium acts on a half-life so the trick is to keep the dosage even - not so much how fast it works), and both have kept me "even" for 20 yrs....
The only reason I prefer tablets is that the total # of tablets I take seems smaller since they never fill up the capsule. That, and a faint smell from the capsule (which I may be imagining since I've never heard anyone else comment on it!).
A good question!