Everyone here has got great points. I second all of them. The thing is, with 7 days, it might be better to just do Bali. And I also agree with linda about staying in Ubud and then Seminyak.
Maybe you could check on the weather to help make up your mind as well? April should be the time where the seasons change - form the NE to the SW monsoon, so you might want to check that up with which part of April you're planning to go, though I THINK (not sure) that you are quite safe, and if you are, and you want a really nice beach, then, while I love Bali, I do think that Thailand's beaches are much nicer than Bali. Then again, I am very fussy when it comes to beaches. I only like white "powder" sand and crystal clear blue waters.
I have been to Bali many times in April and have always had good weather but I can't remember if I have been to Thailand's beaches in April or not.
Just my 2 cents'....