I know this is an old thread, but I thought I'd add something.
The first quote, about solvent based sealers being more effective for stone because they penetrate more, is a myth. It's true solvent based sealers penetrate more than water based sealers, however, this does not mean one is necessarily more effective than the other, generally speaking. It depends on the application. We are only speaking about carriers. When the solvent or water dries, the solids left behind is what matters.
The way I like to explain it, is that water is larger than solvent. Everyone has been told this. So theoretically, you would want to use a water based sealer on more porous stones like limestone and some granites. Contrary to popular opinion, travertine is not all that porous, unless it's in a tumbled form. So using water based sealer on limestone, it would clog up the holes faster, closer to the surface, Thus holding the sealer and allowing it closer to the surface longer, allowing the solids to dry close to the surface. With a solvent based sealer, you would apply more and more applications because it is penetrating deeper and you are building off that first "layer" inside the stone.
one really good solvent based sealer for porous stones is Stone Pro Porous Pro. It's one of the best I have ever used, and it's cheap.
One sealer I have liked for a while is MB-4 from MB Stone Care. It is water based, but acts like a solvent based in the fact that it is easy to clean up the excess and it dries fairly quickly.
Dry Treat Satin proof is a silane based sealer. So is Prosoco SLX 100. The SLX 100 has done as good or better for me on everything I have put it up against, and I have done a lot of head-to-head with the two. They are designed to bond with silica, so they do not tend to do well with marble, travertine and limestone (MTL) surfaces. They excel with sanded grout, concrete, granite and sandstone. Dry Treat is from Australia, so you're paying a premium. Prosoco is made in the US and they are about 2/3 the cost, with the same or better performance.
The major benefit to the silane sealers is you can literally spray it on and walk away. I always like to spread or push it in with a flat mop, but no buffing is required.
MB Stone Care has just sent out test bottles to some people, of their new sealer. I have a bottle but haven't tried it yet. It is a new water based, VOC-free sealer. If it's like anything else that comes from MB, it should be great.
There are different sealers for every application. Certain climates and regions will allow sealers to work better than others. You must test to find what works best for you.
EDIT: One important thing to keep in mind is that to truly seal a surface properly, when using multiple applications, is to let each application dry and let the stone return to its natural color before moving on. Some stones will take 30 minutes. Using the incorrect sealer will require many applications, thus a lot of down time.
One coole product we have (This was an after thought, no intention of intruding on Larry Cobb here) that not many people are aware of, is F-721. This is a concentrated sealer that comes in a pint. You mix it 7:1, with water OR solvent. Depending on your needs and your application you can carry one bottle and have a solvent or water based sealer.
http://www.easystonecare.com/F721-Premium-Fluoropolymer-Clear-Sealer-p/cc-f721.htm
