Hi,
Kinda new here, and not a Moog expert by any measure. I don't even own a Voyager yet (though I did order one recently). But I have been researching this exact topic, and this is my impression.
The Voyager XL is not an entirely different instrument than the regular Voyager. And if you add in the CP-251 and the VX-351, you really are something like 95% of the way two having an XL, in terms of capabilities. So it's not that an XL makes sounds that are just clearly and obviously different from anything a standard Voyager can make. It's more about what you want or plan to do and how much time you want to spend setting it up vs playing it.
So, one obvious difference is more keys. Which of course you can get with a bigger controller.
Another obvious difference is the patch points. Which of course you can get with the CP-251 and VX-351.
There's the ribbon controller, but you can get a 3rd party ribbon MIDI controller.
Another apparent difference which I have read about (though hard to verify) is that the XL has additional parameters for LFO2. You can read another person explaining this in the 7th post in this thread on another forum:
http://www.gearslutz.com/board/electron ... 351-a.html
The XL isn't backlit like the Electric Blue and Select series, so I guess you could call that a point against the XL. Unless you're going with the Performer Edition. And if money is a big concern, you're probably looking at the Performer Editiion, so in a way that point is kind of moot.
But think about all that. You're comparing one monolithic unit compared to a synth + larger controller, + two add-on modules (probably including a rack and rack cabinet unless you want them laying on a desk or something) which also involves the interface cable for the 351 + an additional ribbon controller, and 2 or three power cables (not sure if the VX-351 has it's own power or sucks it in through the interface cable). That's just a lot of stuff. Not so bad if you plan to set it up in a studio or a bedroom and leave it all alone. Not so bad if you already own the controllers and rack cabinet and stuff. Though even still, it takes up more room and is harder to move around.
So, the Voyager + add-on route gives you everything the XL has except (supposed, unverified by me) extra parameters on LFO2.
The XL route gives you everything except the cool colored back lighting (unless you're looking at a Performer edition).
Personally, I had shopped everything out so that I had a shopping cart with a Voyager Select, CP-251, VX-351, the rack for those two modules, a 4U rack cabinet to contain them, a stand, a second tier for the stand to rest the cabinet on, a Furman to power all of that stuff, a bench to sit on, a patch cable set, the expression pedal and the FS-1 footswitch. Basically, a from-scratch piecemeal Voyager rig in one shopping cart, and it was $400 less than buying the XL.
Then they announced the $500 off sale for the XL this month. That turned everything upside down. So I just ordered the XL. Don't need the patch cables because the XL comes with a set. Don't need the Furman because the XL only needs one outlet. Don't need the rack or cab. The on-sale XL with stand and bench is about the same price as everything I had selected before. Take money out of the equation and it becomes a decision about convenience and simplicity. Clearly the XL is going to be a lot easier to set up and move than the Voyager + add-ons.
So, if you're not able to buy during the sale, then the question is about whether or not it's worth the extra money to just plug & play. For me it wasn't worth it, until they lowered the price. Now it is.
All that being said, I expect it would be very difficult to point to a specific sound and say "That. That right there is what the XL gives you." At the end of the day, you can only get back-lighting from select and Electric Blue Voyagers. And you can only get the (again, unverified) additional LFO2 parameters from the XL. Everything else is attainable in various other ways.
Also, if I were a gigging musician, and my music was the kind that I needed to do patching and stuff like that on stage, I would absolutely want the XL, just so I could have one instrument, one case, and everything all in one unit. If my music just needed sweet synth sounds, and I didn't need or want to fiddle with patch cables on stage, I'd definitely go with the regular Voyager, so I would still have one instrument in one case, but it would be smaller and lighter.
So, basically I guess it all comes down to personal preference. Like everything else.
EXTREMELY REFRESHING for this great morning 
When I got to number 3, I laughed pretty loud and my coworkers all stared at me. Excellent points. Guess I'm just trying to talk myself the other way since that means saving a little less money, which means less time until there's a Voyager sitting in front of me. Thing is, I already have a ribbon controller and a CP-251 . . . so I'd save a little money, even if I still had to get the VX-351. I'd just be missing the 61 keys, but damnit you're right, "It's a VOYAGER with 61 keys."