Hey Spectre, if i go the jag route I will be buying the '88 XJSC from a buddy who lives in Dallas. If i choose the jag would there be anyway for us too meet up so you could give it a once over? or a general inspection?
Normally, I would say yes. Unfortunately, Thursday afternoon I crashed my motorcycle on 635 so I'm kind of out of circulation for a bit. I can refer you to DFW personnel I trust to look over Jaguars of this vintage, should you wish.
On my own part, I would suggest you get neither car. The XJ-SC is actually not a bad car, but it has one big problem - that targa top leaks and leaks quite badly once the cars get older. I remember taking one for a test drive a few years ago on a sporadically rainy day and wondering why I had thought there was a problem with the XJ-SC. The liter or so of water that was promptly dumped in my lap during a cloudburst a couple minutes later reminded me why.
There was no good permanent fix for this except for one guy who was making a replacement targa top of a new design for IIRC about $4000. I have no idea if the guy is still making them or even if he's still around as it was a few years ago. I would suggest getting a later actual convertible XJ-S and skipping the -SC. Here's a better local option (though you may want to spend the money to fix the top - converting to manual operation is common when the hydraulic lift motors fail, though those are relatively easily replaced if not the cheapest thing)
https://dallas.craigslist.org/ftw/cto/d/1990-jaguar-xjs-convertible/6352317455.html
The 03 S600 is a prime "Mercedes Rot" car and will have massive electrical problems. While it doesn't have the biodegradable wiring harness of earlier models (yes, Mercedes made a biodegradable wiring harness and exposed it to the outside where it, uh, promptly biodegraded) it will still have a troublesome ABC system and other fun electrical issues that can cripple the car. The V12 engine bay is particularly tight and multiplies servicing costs; add to that the additional costs of the low production volume V12's parts and the heat of the V12 shortening component lives. It will also have the failtacular 5G-tronic automatic transmission that can cost $10-15K to repair. If you have the money to pour into it, I suppose it'd be fine, but be warned that thing is a black hole into which you will pour money.
