Welcome to the forum, Frieda.
A penetrating sealer is one that soaks INTO the tile and leaves solids in the pores. Once dry, it isn't visible...you can't see a real difference in the tile color.
An enhancing sealer soaks INTO the tile and will bring out the color in a similar fashion to the way wetting the tile does. However, it doesn't leave a glossy coating. This sealer also soaks INTO the tile. If you like the color of the tile when it's wet, this is probably the best option for you. But don't forget that there will not be any real sheen to it once dry.
If you like glossy, then you'll want to use a topical coating. This sealer sits on TOP of the tile and imparts a gloss. I think this is the least desirable sealer because the coating is susceptible to wearing and this degrades the quality of the sheen in a big way. A topical coating requires the most maintenance to keep it up.
So, probably your best option is to take a sample tile an apply any sealer you're considering to it and allow it to dry so you see exactly what the tile will look like. Realize that travertine is a natural material and if you apply an enhancing sealer (vs. the penetrating sealer) the color will vary from tile to tile...in some cases, dramatically.
Typical protocol on sealing natural stone is to seal the face (and the face only of the tile, being careful not to get it on the sides or bottom of the tiles) once before you grout, then a second time after you've grouted. This is for a couple of reasons: 1) the sealer helps prevent the grout from soaking into the face of the tile thereby helping you clean off the grout haze during the grouting process; and 2) the act of grouting can be pretty tough on the sealers and it's a good idea to seal again after the grouting.
...However, using an enhancing sealer (vs. plain penetrating) on natural stone tile can get a little tricky because you have to contend with the grout. Using an enhancing sealer over grout darkens the grout....which might be okay if you choose an artificially lighter color that will darken to your liking, but there's a common problem of the grout taking on the sealer unevenly and becoming rather splotchy. That splotchiness isn't predictable. And the splotchiness is permanent, unless you regrout. If you're okay with both the grout darkening and there being a chance for splotchiness, then do the normal protocol of sealing once before the grouting and a second time after grouting.
But I don't like taking chances, so if I am going to enhance natural stone, I like to do it the day before grouting, then be done with it. I don't want to take that crap shoot on the grout darkening evenly, or very splotchy. So I don't enhance after grouting. If the act of grouting does strip away enough of the enhancing sealer to lighten it's affect, I'll carefully seal the tiles individually with a foam brush, taking great care to not get the sealer on the grout. This also brings up another reason why enhancing natural stone is tricky...if you're going to avoid sealing after grouting, the grout will receive no protective sealer. So it's a good idea to use a grout that has built-in self-sealing qualities or to use a special additive like "Grout Boost" that imparts these sealing qualities.