I would respectfully say that, if you have little experience on either a keyboard sequencer OR a computer one, DON'T waste your time on the keyboard one!

Firstly, as an admitted 'newbie' keyboard player, you are going to make a LOT of mistakes in your playing. All these will need editing to make correct. It is FAR easier to do the editing on a computer sequencer, with it's huge graphical display of what you played, than a tiny display on a keyboard with only a list edit to work with (or a crippled and hard to read graphical display).

Next... add in the fact that you, as a guitarist, are going to want to record your guitar playing (and singing, and anything else audio) so will HAVE to have a DAW, most probably a computer one... So why not combine the two recording systems, rather than have it all split up (twice the stuff to learn) between two places?

An entry level MIDI/Audio package like CubaseSL, Logic Express (if you are on a Mac), or even LiveLE (if you want to use a lot of loop production) would integrate all your needs, AND make it easy to use all the sounds in your computer....

Finally, I would say a nice arranger like a Roland E60 or Yamaha S900 would provide you with all the keyboard stuff you need outside the computer stuff, and both are capable of generating backing tracks that can be saved as MIDI files, transfer to the computer DAW/Sequencer, and finish off the composition there. This is how most people that do full production work that includes an arranger starting point usually work.

Best of luck with what you chose. I have been doing this for at least fifteen years with this kind of rig, and there is NO WAY I would ever hobble myself with an on-board sequencer. It would slow me down immeasurably...