I've been both to Plivice (twice) and to Krka. They are different kinds of parks. Krka is bigger and more spread out, with a few highlights far apart. One of the highlights of Krka is a huge, magnificent cascading waterfall called Skradinski Buk that dwarfs anything at Plitvice. You can walk up and around this waterfall - it's lovely. You can even swim below it (even in May, a few people may be, but it will still be cold). You can't swim at Plitvice.
There is another falls, some Roman ruins...but you can't easily walk from place to place in a short time. You can drive between them or take day-long boat tour, I believe.
Plitvice is a much more cohesive hiking experience than Krka. There are dozens of little waterfalls, sometimes at every turn. It's amazing - but also extremely crowded at times. It's a developed park with wooden boardwalks. You can take a 20 minute boat ride between two parts of the park, the Lower Lakes and Upper Lakes. Otherwise, you're just hiking (you could also hike between upper and lower or take a shuttle "train" - just a motorized tram). At mid-day it can be uncomfortably crowded with day trippers.
If you visit Plitvice, I'd say it's essential to stay overnight at or near the park and enter when it opens at 7am, to have a few hours without the big tour groups. You could do two half days and a night - get in late the first afternoon, see part of the park (day trippers should be leaving by late afternoon) and see the other part the next morning and plan to be out by noon - or just prepare yourself for more people after 10am. There are a few hotels within the park itself (operated by the park ) - they are overpriced for what you get but convenient. (I stayed at one once.) Book one of the park hotels early - I hear they now fill up early with tour groups. (Maybe on Booking.com where you can cancel your reservation without penalty later if you change our plans.) Otherwise, there are plenty of sobe (B&B's) near the park but not many conventional hotels if any.
If you want a completely different experience from all the tourists at both Krka and Plitvice, try Una national park in Bosnia not far from Plitvice. It's a very back-to-nature experience; the park is fairly new. Like Krka it has a huge waterfall - but I guess you'll see a big one at Krka anyway. If you just want to see another big waterfall, you could detour to Kravice Falls not far from Mostar. Easy to drive to - I was able to drive right up to the falls, but at high season you have to park in lot and walk in or take some sort of shuttle bus.