All nuke spots are fairly competitive. You'll have to jump through the hurdles of the Naval Reactors interview which isn't a walk in the park. As far as how everything else compares, they're a bit like comparing apples and oranges. Nuke SWO you have to do a conventional Division officer tour first, then nuke school, then a carrier tour. Subs you go straight to nuke school, then sub school, then your division officer tour. The life onboard each ship is completely different. It really comes down to what platform you want to serve on instead of which is better/easier to get into.
