Synth lead lines don't need ultra fancy complex sample type $*^t (generally that is - you can use whatever floats your boat - YMMV). I find subtle combinations, modulations, and manipulations of simple waveforms to be most useful. Anything with easily editable parameters would be a good choice. SE1 or Nord Rack are 2 that would complement your planet phatt nicely.
I played the Karma for about 30 mins at a GC the other day. I thought it did some very interesting things with the control, sequencer, arpeggiator and patch. I didn't see anything on it to make me think it would be a great lead synth though.
I've done lots of demos for E-mu's past few modules like the B3, Virtuoso, Planet Earth, etc. I quite like the flexibility of their synthesis engine with multiple legato modes, filters and modulation routing. I say all this because, not having played the Mo'Phatt, I can't say how it would work. However, to me, it seems like its synthesis engine would cope nicely with analog 'simulation'.
I was thinking in terms of your original post about "hip hop and RnB melodies". To me, that means single oscillator, simple waveform, monophonic, portamento and very little modulation. This ISN'T what I heard in the Karma. I heard very complex, arpeggiated, modulated waveforms which were very interesting but had nothing to do with "hip hop and RnB melodies".
To do "hip hop and RnB melodies" I'd get something analog or modeled analog. Nord MicroModular, Microwave XT, Micromoog, etc.
Then again, I might not be connecting with the kind of sound you're looking for. Perhaps a few examples would be helpful.
I think Nate and I are thinking along the same lines. I made my suggestions based on their on-board controls. I can get a nice lead synth sound with either of those in a hot second, because everything you need is right in front of you. I shy away from preset type units for the lead tracks, cuz it takes more time to search thru presets than just to make one myself.
IMO, the Karma and the Mo'Phatt would be counterproductive for that application.
I just went through a big keyboard purchase myself! If I were in your position, depending on how I felt about the Planet Phatt, I'd either keep it (if I found it indispensable for my work) or possibly trade it for the new "phatt" module (if you feel you need to "update" your sound). Having two modules from the same company, that do basically the "same" thing can lead to a pretty "homogeneous" sound. I'd go for the Karma because if nothing else, it'll give you some different colors.
Personally, I was trying to decide between the Mo Phatt, the Korg Triton, and the Roland XV-3080 (I do mainly pop/R&B stuff, but some other things as well). I didn't really look at the Motif because I've never much liked the user interface on any Yamaha gear I've owned or used, although everyone I've talked to that uses it thinks it's great. I ruled out the Mo Phatt first because I knew I would burn out on it quickly. Too much of one thing IMO. I really couldn't decide between the Triton and XV-3080. I LOVE the sound of the Triton. Big, fat, warm, clear and exciting. A few too many pad sounds for my liking, but awesome sounding. The XV-3080 didn't excite me at all in the store, but seemed to have an awful lot of range. I bought both and left the store thinking the Triton would be my main thing and the Roland would fill in the holes. I've since put the "Vintage Synth", "Techno", "Hip Hop" and "Special FX" JV-series cards into the XV-3080 and I've got to tell you, that Roland module is AWESOME. Probably the best overall piece of keyboard gear I've ever purchased (and I've got a room full of keyboards over here including a lot of the "classics"). And contrary to what I thought, I use the Roland for EVERYTHING (leads, basses, pads, pianos, drums - everything) and the Triton to fill in the holes!
Bottom line is, you can't go wrong with the Korg Karma (same sounds as the Triton). My Triton is amazing - wouldn't want to work without it. But just to confuse matters even more, maybe you might want to check out the XV-3080 with a couple of the cards? Hope this helps.
Oddly enough, an E-mu MP7 (tabletop/control surface version of the Mo'Phatt) showed up on my doorstep yesterday. I'll be working with it for the next coupla weeks so I can give you a better opinion of it if y'all like.