Rhys and Clord,
here's my two pennorth, as one of those crusty 40 year olds who has "worked his way up" to big bikes over 22 years. On the way I have had big scary crashes on a ZZR 1100 and an R1100GS and decided that I owe it to myself to either get good at riding or give it up. I have ended up taking up racing, and getting more out of my biking, even on short journeys around town, than I ever thought possible.
If you like your test rides - and you should get a half day one from a BMW dealer - then go for the GS. Although big it is very easy to ride, the ABS might save your life, and when (not if!) you drop it you'll find it very tolerant of crash damage, and cheap to fix. (To give your body the same qualities, you MUST of course be covered from top to toe in CE body armour on all journeys, however short. But I'm sure you know this!)
BUT - and please don't take this the wrong way - continue to regard yourselves as learners. Most people don't bother with training after they get their test - but the fact is that you can get to an "expert" skill level in a year if you work hard at it. If you don't it'll take 10 years or more...and you won't get half as much fun out of your GS. Invest time and money in training and you can quickly make up for all that time you've "lost" by not starting out as a spotty yoof.
My ideal curriculum would be:
- join your local IAM group and get your advanced test. This will teach you defensive driving, good cornering lines and bold but safe overtaking. And you'll get cheaper insurance.
- do the BMW off road course. You will lose your fear of slippy surfaces and find out the right way to pick up a dropped bike. (Very easy, even for 5'9" shorties like me.)
- do several levels of the Keith Code Cornering School or the European Superbike School, or both. You will learn how to attack corners like a God. And you'll find out how huge your reserves of grip are. Practice what you learn on these courses by doing track days
- do a course at the Nurburgring with the Nurburgring Riders Club. With luck it will rain - you will become a rain master, which is immensely satisfying as well a life saver. Plus you'll learn how to use ALL your braking powers in an emergency. This alone is priceless.
All of these courses are enormous fun as well as great ways to extend your life expectancy.
The above will make you master of the open roads. The one thing you won't learn is how to be safe and fast in the city, courier style. If you want to go at courier speed, you need to practise, practise, practise. Or get a job as a courier!
Hope this is helpful and doesn't seem too patronising.