From the start, this has to be seen as being biased (at least honest!), because although I've had full use of a 600 for about 2 weeks, that's not living with one day to day all year round.
I chose the thou gen 1 for the 4th time for a number of reasons.
It is imo the best all-rounder (at the price you pay) there is. It will tour in reasonable comfort loaded up with luggage, and still handle reasonably well with that load. It can tackle the twisties and deliver a smile whilst doing so. It's a brilliant bike for fast sweeping A roads. It's surprisingly nimble for town work. Fuel consumption is very good, provided you keep it below 7k rpm, and it will pull cleanly from as little as 2k rpm in any gear, once it's little idiosyncrasies have been sorted (more about mods to follow). Every time I've bought one, I've considered other bikes and come to the conclusion that while there are plenty of bikes out there that will do one or two of these things better, I cannot think of a single one that can do all these things as well in a single package as the Fazer thou does. This means that as an only bike, it can't be beaten. If, like me, you cannot afford something newer, it is now superb value for money. It is generally reliable (worst I've had is a collapsed rear wheel bearing, this on a bike that spent it's first 20k miles based in the salty air of Guernsey). I've thrashed em, toured em, done fast roads, gravelly, dodgy surfaces washed out with rain on slopes, both up and down, that were more akin to waterfalls in pissing rain, and always they feel sure-footed and confidence inspiring (except on original fit MEZ4s!).
As can be seen with the bikes of Kosmic, Devilsyam and others, it can be made much better even than this at reasonable cost (comparatively). Many 600 owners like to upgrade their suspension (notably with the rear shock from the thou), and the thou will take R1 USD forks with well established set-ups, R6 rear shocks too. With the Ivanising carb tune-up, all the little glitches disappear and you get more power everywhere, again at relatively low cost. As with the 600, there are enough around that spares, whether new or second hand are generally readily available. AyJay has proved the general reliability by taking his around the clock, and it's still going strong.
And don't tell me they're not fun! Ok, the 600 is bound to be more flickable with it's lighter weight, but the thou will get round most corners just as quickly in the hands of a good rider, and then stay with many a sports bike on the straights too. And you can't beat the grin factor of all that mid range, power where you can readily use it, not all stuck up in the last couple of thou revs before the redline. But you want to rev it? Yeah, you can do that, it just takes a bit more care and forethought if you're not to get in too much trouble. I love engine braking into the corners, flick it in, and then powering up thro' the gears. It's far from impossible. But not feeling so energetic today? Hell, leave it in top and virtually do your entire trip in the one gear.
Given time, I'm sure I could come up with plenty more, but 600 owners probably wouldn't read it anyway!
Don't get me wrong, I'm not saying the 600 is a bad bike, in fact it seems to be the equivalent of the thou in it's own capacity class. But if you want to do more than one thing on your bike, and you can only afford one bike, I'd say go with the thou. Every time.