They both have their merits.
UE Watts are designed to be more realistic, where as IC2 EU is simple and effective. UE has some very fun stuff in its net, where as IC2 as well, but they differ in how one goes about using them. UE is a API, where as IC2 is a Mod. There's where the difference lies. UE is designed to provide a realistic electricity system for modders to use. It was designed in the begging as a alternative to IC2 power due to the IC2 API EULA and Calclavica not wanting to agree to it's terms. So UE is mostly used by people wanting to not agree to the IC2 API EULA. It's a matter of taste to most modders, but to ones who read fine print and whatnot, UE is a somewhat better choice. Most IC2 addons are small, or not fit for inclusion into IC2 itself(Yay GregTech!), but if ICBM was built within the IC2 EULA, you could bet your 2 stacks of UUM that it would be in IC2 within one release.
But in a Nutshell, Hazelnut. In short, UE is not anything like IC2, because UE is just a API, something many people forget. One could use parts of UE in your own mod without much effort or even fuss. But IC2 is a big mod, with plenty of different aspects, some of which many people use(Nuclear Reactors), and some almost no one cares about(Crops, or scrap boxes). A good thing to note is one can't(assuming it hasn't changed since I last checked) use a Monetizing link for your IC2 addon, but one can do so just fine with a UE mod.
Things I enjoy with UE: ICBM, Atomic Science, BllastCraft(I like being able to see through my windows without worrying that they will break if a creeper shows up, and very few mods have such durable glass blocks), Power Suits(used it a bit before it was on the IC2 forums, loved it), and some parts of Mekinisim(mostly the powered bow. I need a energy weapon that isn't a mining laser).