I've used all three, and to be quite honest, if you don't want to spend any extra money on recording software, and want to spend money on Plugins instead, go with ProTools. It comes with the interface you buy, and I think is the best software you can get that is coupled with hardware. However, because you can only use it with your Digi console, if your power supply poops out (common 002 problem) and you're in the middle of a session, it can be a pain in the ass to wait around until it gets fixed. And beware the default plugins, especially the reverb. Gross.
I've done quite a bit of work with DP, and I like its plugins a lot. They're definitely usable, and don't leave many gaps to be filled by third-party plugins. And if I remember correctly it comes with CueMix Console, which is a monitering program that is VERY helpful for routing signals and level adjustments to individual outputs, like setting a specific mix for the headphones that is different from the one you hear in the moniters.
Logic is pretty good, but I find it honestly to be a bit overrated. My friend bought it and we've done a lot of playing around with it, and it just doesn't feel right to me. I think it actually might just be that I think the color scheme is too dark, being used to DP and all. But I've been mostly unimpressed with the MIDI instruments that come with it (even with the right EQ and verb, etc, it's difficult to make some of the instruments sound real) and that's mostly what you're paying for. Although the Rhodes and Hammond VSTs are AMAZING. And some of the synths are pretty cool.
They all basically work the same. They all have similar layouts, similar interfaces, and similar functionality. Some come with better plugins, but if you plan on purchasing plugins anyway, go with something that is cost-effective for what fits your budget. You can basically pick any of the softwares and learn to use them effectively.
My personal plan is to buy an old Mbox or an Mbox 2, just to say I have ProTools in case someone wants me to use it in their session, and to also have a larger interface with either DP or Cubase for the clients who trust my opinion. I hope that helps some.