Yahoo is good. But Sleeper is better for Dynasty/keeper leagues and has better mobile functionality. Their platform is mobile based and I will agree that they need a better iPad app but the formats are better as far as drafting and layout. Yahoo can lock up with ads sometimes as well.
I agree, do not switch just to switch. If you have a dynasty or keeper league Sleeper is better as it does not shut down in the off season and you can continue trades and other league discussions. Yahoo may catch on at some point.
All-in-all, I lean Sleeper as I find the overall functionality of higher quality. I do have one league still in Yahoo and have no intention of trying to push it to another platform as it is purely redraft.
Hope this mud is clear…lol