One thing is you didn't say is what software you have virtual instrument /FX wise.... I don't know off hand all the tools you have and what they come with virtual instrument / FX wise.. Usually, obviously enough.. whatever comes with one DAW type software.. you can't use in the next DAW.. rewiring options being a pseudo exception.
It's also not clear to me why you need all that or what level you're at with this stuff... and there are two basic issues that could impact your thinking relative to this stuff.
#1 When you're just starting out it might very well be that what comes with the DAW in question is good enough for you.. for some period of time... At the level I'm at.. I tend to assume, often erroneously, that what comes with the DAW "isn't good enough," so I mostly rely on a plug in folder that goes with me from DAW to DAW.
#2 There's a danger when getting too many DAWs.. that you don't get really masterful with any of them. I think this is especially an issue when you're starting out.. I've been making music on computers since the 90s.. I now mostly just move between Digital Performer, Reason, and Ableton... and because I move between these programs I'm not as strong in any given one as I could be.. so even with a lot of experience there's trade offs.
I remember reading this book from Berklee press that suggested "never buy a new synth until you've mastered what you already got.. and though that sounds like crazy talk give how things are today.. I think there is still huge value in limiting your tool set.. and focusing on a few tools. (Advice I don't follow AT ALL but still probably good advice)
I don't have an iPad.. but the most basic version of a new one is $500, right? I mean not including the new smaller ones? I don't know if the $500 model makes sense for the amount of space on it but..
Well here's my picture.. I see there as being a continuum between between desktop, laptop, ultra portable laptop, tablet, and smart phone.. and I think there's value in having something that fits into all of these categories.
I'd mostly echo what other people are saying.. that I think the biggest draw of the tablet / iPad is that you interact with it in a way that's different from a regular computer.. plus there's a different form factor.
I want one to be able to do the Jazz Lumer type stuff / control for Ableton / regular DAW.. and I don't really have a lot of expectations beyond this.. music making wise anyway.
I also have the impression that there's a very short shelf life on iPad's and iPhones.. although that probably doesn't effect using it as a controller as much as it would in other areas.
If you're thinking about adding Logic and ProTools... I don't know studio one
well enough to say this.. but I might drop Logic.. for an iPad... an iPad might have more value for you simply because it takes you into an area that you don't have access to.. where as with your DAWs you have a lot of overlap.