A tablet is nice because it's a bigger screen. I like reading ebooks on them. (plz don't tell me to get an e-reader)
A watch is nice because it means you can ignore your phone, but still reply to notifications. I like the more accurate step count. I'd really like a device that counts my lengths when swimming too, but I haven't had a chance to use one yet.
In both cases I've gotten devices that have more battery than I expect. I rarely found myself in a situation where charging was a hassle.