The thing about using a sampler/sequencer inside a keyboard is that your display is only so big, and the amount of available sampling RAM is limited, even by the most powerful synths. On your Motif, should you buy one, you are looking at a screen that's only about 4x2 inches. It can't compare to the ease of use when you have a nice big computer screen in front of you with a great graphical interface.
With a decent PC you can run software samplers that will allow you to load of sound libraries that are many times larger than the 64MB limit you will get on the Motif. You will also be able to manipulate things graphically with your mouse. You can "slice" up audio and stuff, and move things around so easily, without really having to learn as much because the GUI makes things much more intuitive.
With sequencing, the PC is again far superior to any keybaord based sequencer, for much the same reasons. I use Cubasis, which is a cheap, scaled-down version of Cubase. It was only about $90, and it sequences both audio and MIDI.
If I play in a 16 measure piano part, for instance, that I pulled off flawlessly save for a single note somewhere, it can be tedious trying to find and correct that note in the internal sequencer. Otherwise you would have to play it again. Most software sequencers have something like a "piano roll" where you can open the recorded MIDI part on the screen and it shows you where each note begins, ends, the velocity, etc. for every note. It is incredibly easy to understand. Missed that Eb and ended up with an E? just grab that "note" and move it. It's SOOOOOO much easier.
Anyway, if you haven't thought about any of this, you should, The sampler in the Motif is great, and I am glad it's there, but I don't ever use it. There is simply no need, unless I wanted to play live somewhere and needed to load up a few samples. 64MB isn't exactly going to get you too far these days.
I do use the sequencer a lot for capturing ideas quickly, but when I want to do anything serious, I use the PC for sequencing.
One more thing, actually. When using the PC like this, I can record all my MIDI tracks, then record them to audio one at a time, so I can use all the available effects for each voice. When using the internal sequencer, the Motif is limited on the amount of effects you can use. When using the PC to turn my MIDI tracks into audio tracks, I can use a different insert effect on every track if I need to, which allows the beauty of the Motif's voices to really shine through.
The Motif rack is only $1,000. Assuming you were just going for
the Motif6, you are saving about $800. That $800 could buy you a good software sampler, a sequencer and MIDI controller.
I don't mean to offend here, but it sort of sounds like you are not 100% sure what you are getting into here. You could get a program like Reason, and a very nice MIDI controller, for less than $500. Reason has it's own sequencer, it's own sampler, and several synths. It has some really incredible sounds, and can be used for any type of music. This might be right up your alley.
I could probably type more, but I am going to quit here. What type of music are you looking to make, anyway?