If you search on Google Paul for Jaguar S type v Rover 75 there are a few articles to be seen , this one is quite good and the bottom line seems to be that the Jag may be slightly nicer but is a lot more expensive to repair and then of course there is your extensive knowledge of the 75 and the network you also have available if you run into problems you can't solve yourself which all adds up to cheaper motoring
Planning is an unnatural process, much better to just get on with things, that way failure comes as a complete surprise instead of being preceeded by a period of worry and doubt
My answer may not be terribly halpful, but I'll post it anyway!
Autocar did a comparison between a very early top-spec 75 (2.5 Connoisseur SE) and a very early bottom spec S-Type (3.0 V6, not diesel, sorry) in mid-1999.
They said that there wasn't much in it, but the Rover was slightly better. Of course with higher spec S-Types you would expect a different result, but shows that they are not too far apart.
Apparently later S-Types are far better, generally the opposite seems to be thought of the 75.