Something I think is really worth considering are the third-party equivalents.
Sigma make a 24-105 f/4 IS which is far, far sharper than both the Canon ones, has more accurate transmission, and the IS it uses is about on par with the Canon mkII. It's also one of the few Sigma zooms which doesn't have any particular AF problems. The only downside to it compared to the Canons is it's not weather-sealed in any way, though it's built fairly well and it doesn't sound like you're taking it up mountains or anything like that.
The Tamron 24-70 f/2.8 VC is also worth a look. You're losing some long-end, obviously, but you're gaining more than twice the light in terms of actual transmission (which will also help your camera's AF system on top of enabling you to keep the ISO low) and at f/4 it is far, far sharper than any either of the Canon 24-105s or the Sigma 24-105; shooting the Tamron at 70mm and f/4 and cropping will get you sharper details than zooming in to 105mm with any of the other three lenses at f/4. At f/2.8 it's about equal with the Canon 24-105 mk I is at f/4, so compared to what you currently have, you're not missing out on anything; at f/2.8 you're gaining light with the same IQ and at f/4 you're gaining a lot of IQ and still gaining a little bit of light over either of the Canon lenses. The VC works very well, unlike the Sigma it is weather sealed, and though AF is just a tiny bit slower than the Canons, it's equally as accurate. Tamron recently released a mk II ('G2') which improves the optics, VC, and AF further, though also costs quite a bit more; the mkI/G1 can be found very cheap now and really it's 99% the same.
I know a lot of people won't even consider third-party lenses, but when it comes to this standard focal length, Sigma and Tamron both do a really good job.
As for the Canons, it's a really tough call. Value and "worth it" mean different things to different people; a price that doesn't bother one person might be far too much for another. Nobody can tell you whether the mk II is worth the money because we don't know what your income is, what your lifestyle is like, and what that money might mean to you.
However, from a purely technical point of view, it is very hard to recommend the mk II over the mk I. The mk II is heavier and a little larger, which is a problem for a general purpose lens. The light transmission is a little better, but it's still about a third of a stop worse than it should be. At the wide end the corners are good but the middle is softer than the mk I, and at the long end the opposite happens. There's less distortion at the wide end, but distortion beyond 50mm is unchanged. The AF and IS are both a little bit quieter but not significantly so. The IS is improved by about one stop (though this is a tough one for me to judge as I'm very used to holding much heavier systems still at low shutter speeds without IS, so I can always push IS a little further than some people seem to be able) but the AF is the same.
In other words, optically it's basically a draw, and the electronic improvements come at the cost of size/weight.
To me, that makes it dead even. If someone doesn't have the mk I lens then they I'd say they should definitely go straight for the mk II (assuming they can afford it), but when someone like yourself already has the mk I, it really doesn't make much sense to bother, to me. Having used both—as well as the Sigma and Tamron I mentioned above—I struggle to tell the two Canons apart. For a general-purpose lens, I'd say the mk I is better simply because it's that little bit lighter; for professional studio use I'd say the mk II is better due to the lower distortion at the wide end. Even those scenarios are splitting hairs.
