Actually, they're both pretty nice machines, but which one is right for you probably depends on what you're going to do with it. As a versatile, all around machine, I'd say it's just about impossible to beat the GS6000. But there are some caveats I'd throw in. First and foremost is that the version of Colorburst that ships with it is--to put it very charitably--not terribly robust. Profiled correctly, the GS6000 is the largest-gamut solvent printer currently on the market. However, it's just about impossible to get all its capability using the cheesy little RIP it comes with.
Now if you wait long enough the prints on paper are really nice, its just the problem of the ink drying. We have tried different papers but all got this problem. And also nobody seems to know a solution.
I profile GS6000's to print on paper all the time. They do real well at it. There are some papers that don't work real well with solvent ink, but keep in mind that the ink the 6000 uses is the same ink Roland and Mutoh use, and there are plenty out there--the ones from Sihl come to mind--that accept this ink with none of the problems you describe. If you're having an over-inking issue, it's a profile issue, not the machine. (Of course the 6000 is still solvent. It's never going to match the 9900 in terms of absolute image quality on hard-surface media.)
The 25500 on the other hand, tends to be a great machine for printing vehicle wraps. If you needed to dedicate a machine simply to print on cast vinyl, I'd give it a very hard look. However, due to the fact that it only dries ink with heat, it has a very, very narrow range of ink limit settings that it will tolerate on various media. And if you overink with it and the ink comes out uncured, it will never, ever dry.
I think for that reason, many of the stock profiles I've seen for that machine tend to be on the weak side, inhibiting what's a surprisingly large gamut if the machine is profiled to its full capabilities. Also, it's possible to get some weird effects in areas of heavy ink coverage from these machines where the visual effect is a loss of glossiness, and there are some media that simply don't want to accept the latex inks at all.
So for my money, bottom line is I've been a fan of the GS6000 for awhile--provided it's run with a real RIP--and I'd still recommend it over the HP as a general-purpose printer, but the HP does have some advantages. It just depends what you're going to do with the machine.
Mike Adams
Correct Color