I would say that reactors are meant for short, medium, and long term use, however solars are meant for very long term use.
for "very long" you'll need wind, as you'll need lots of EU for a bigger base and solars are just inefficient in terms of space.
you also have to consider that reactors don't just produce a lot of EU/t, but also a lot of EU per cell in total. if your not wasting EU by letting it run while your storage is full or you're producing UU-matter permantly it takes some time to drain all of your available uranium and if you're producing UU-matter as common resource solars won't cut it anyway