Thinking of making the move from 1200GS to either a Bonneville or Street triple. Opinions from those that's done it welcome please.
I haven't done it, but I do have a Bonneville, been tempted by a Street Triple and owned (still do) too many BMW's. The answer really depends on the riding you do and what you want. The R1200GS, Bonnie and ST are all very different bikes, and the best bike I've ever owned is probably the least suitable for the riding I do now.
I did fancy a Street Triple but after getting the chance to ride one last month, I'm now cured of that. Nothing really wrong with the bike, but it simply wasn't for me. I bought the Bonneville at Christmas and so far it's the bike I've ridden most this year. Not because it's the newest, but simply because it's the easiest, is fun and has fitted in better with rides with friends than some of my other bikes. I've got to say that I've spent quite a bit on it, and enjoyed the tinkering, getting the bike to how I want it, and if I had to choose just one bike to keep at the moment it would probably be it, though letting some of the others go would be hard.
The BMW's without doubt are 'better' bikes. The feel, the fit, the finish, the performance, brakes, economy, etc are all better than the Bonneville. But, for whatever reason, mostly this year it's been the Bonneville keys I've reached for, whether for running about or for a trip over to France. It's far from the best bike I've ever owned, but it does have a certain character and ease of use and simple honesty that's endearing.
The Street Triple on the other hand is a modern bike that really begs to be ridden hard, sure it's docile enough around town and tootling in the countryside, but in that environment, my wife's F800R is comfier, more relaxed and has a better riding position. Anyone coming off a sports bike would love the Street Triple, but for my riding the F800R is much more suited to everyday use.
You've already got a R1200GS, so you know exactly what that's like, and why you're thinking you might want to change. It probably does everything you want, is very competent, will scratch, tour, take a pillion with ease, etc, etc. But, somewhere, something is just a bit lacking. I have a friend who has been going through this for the past two years. His GS does everything he wants, but it just lacks 'something'. He rides with his wife as pillion quite a bit and tours with her and the GS is ideal for that. But for the other 40 weeks a year, it lacks that fun, sparkle or whatever.
His solution has been to keep the GS for what it's good at, he's had it several years, it's paid for and there's nothing wrong with it. The money he was going to spend on replacing it is going into something more fun. While he decides what he wants, he's currently hooning around his locality on a little Suzuki 250 single and loving it.
If you've got to stay with one bike, then you need to look at what you want to do with the bike, what your riding is and where you're riding to. The best bike for you will be the one that best matches your needs. There's not much wrong with any of the three bikes you're considering, but they are all chalk and cheese, choose the wrong one and you'll regret it, get the right one and you'll love it.
I'll be interested to know what you decide.
They both have, I'm sure, many shortcomings but I don't seem to notice them 
