A difficult question. A lot depends on individual taste really.
Either bike would be suitable for the roads you discribe ( though the 'gravel strewn' bit would need caution with either of them) as they much better as back road scrathers than motorway cruisers.
The SRX is probably a bit more 'Cafe racer' style being slimmer and having just the kickstarter but the XBR is also a striking looking bike - the GB500 is really nice looking but you pay a good premium for those looks.
Handling on both bikes is very good, what they lack in top speed on the straights can often be made up through the corners. Performance wise, although the SRX has a 100cc advantage, in real terms they are almost identical - the SRX may have a bit more acceleration but it's marginal. Both will do the same sort of top speed - just over 100mph and will cruise all day at 70-80 if you want. Both make about the same power and both give around 50-60 mpg. The XBR has a bigger tank and you can get 200 miles + on a tankfull.
Reliability wise, the XBR needs regular 1200-1500 mile oil changes to avoid topend wear, electric starter can pack up and the back brake can go oval ( look out for this as it's difficult and expensive to repair and secondhand good back wheels are rare). Front calipers can seize up and need cleaning but otherwise they are pretty reliable.
The SRX is generally pretty good but it has a tendency to blow head gaskets ( or even pull out barrel studs on tuned examples!) and 5th gear sometimes goes. Kickstarting can also be a bit tempremental at times - often blamed on dodgy wiring in the kill switch.
Most spares are still available for the XBR but you can't get genuine exhausts any more. I'm not too sure of the situation with SRX parts but there are a lot of decent pattern parts out there and of course Ebay.
There isn'y really that much to choose between them - both would suit you're needs - just a matter of choice really.
Personally I have an XBR and it's a great bike, but I've also seen some very tasty SRX's......
.....of course there is always the SZR.....
