I have a question regarding to a workstation with keys and a rack version. Let use Motif Xs as a example. What are differences between buying a Motif Xs7 and getting a Motif XS rack and hook it up with a midi keyboard? Other than having keys and not having keys, I notice that a rack version has way less button, knobs and sliders than the version with keys. The screen is also much smaller? So does it mean that it is easier to navigate and edit sounds using the workstation with keys than a rack version? Is it more complicate to change patches and setting?
Also, can I record myself in into the rack version? Are there sequencing program in the rack version?

Let's say I have myself a midi keyboard with knobs and sliders already. Are there any reasons I should consider buying a workstation with keys? Or getting a rack version is definitely a more make-sense choice because it is much cheaper?

The rack version is really mainly used within a DAW enviroment you can use a midi keyboard but navigation is better with the keyed version unless you have the rack hooked up to the DAW. Most people use the MotifXS rack as a VST style instrument within their DAW application. I use Logic and Cubase 5 a lot but I don't have any rack mounts I prefer the control and instant ability to edit directly on the Keyboard so I have a Motif XS-8 and a S90XS (which requires software for editing voices and sequencing but can play midi sequences and edit effects stand alone). If you want a Keyboard that can do everything right in front of you without having it connected to a computer the normal keyed version of the XS is the right choice. If you are concerned abut being able to play it all live easily (MotifXS is a mess live) and don't want to take your DAW setup onstage well Yamaha designed the S90XS for exactly that purpose.

Rack versions are also used by many as a sound expander for their master keyboard, for instance I used a TX802 rack along with my DX7IIFD. Most rack versions do not have the sequencer found in the keyboard workstation version, so the TG77 rack does not have the sequencer found on the SY77. Racks are usually harder to edit from their front panel than the keyboard version, many really need editor/librarian software to make it easier.
Clyde

The keyboard is really two devices, a tone generator and a midi controller, the rack mount unit has just one function so you'd expect the rack mount unit to have fewer controls

What ou need to do is download and read the user manual. They are available on-line. Knowing if the unit has the funtionalty you need is not enough you want to see if it has a usable interface and if you can make sens of the user manual.

My opinion is that a computer as cheaper. I can buy a Macbook for the price of an XS and have $500 left over to buy Logic Pro and an audio/midi interface

You mean the rack version or a keyed version?

And then hope your computer/software doesn't crash in the middle of a live performance or the middle of recording a track. For my needs, I have found it better to take the computer out of my system and work on hardware only, less time spent solving computer/software issues and more time spent on creativity, writing, and recording.
Clyde