I'm so indecisive as to what wheelgun (my first) to get. Why buy a .38 if you can buy a .357 and practically have two guns in one. Is there an advantage to a .38?
Depends.
New, the dedicated .38 is usually a little less money, but often not by enough to be a deciding factor. Used, a good, inexpensive .38 may be easier to find and to find cheap (like around $200).
Often people find that their .357 just isn't as accurate when they shoot .38s out of it. For best .38 accuracy (and many of us find ourselves shooting mainly .38s) you may need a dedicated .38. However, some .357s shoot .38s and .357s just as well. In my case both of my .357s (a S&W 65LS- a K-frame 3" barreled gun, and a Taurus 605- a 2" small framed snub) shoot much more accurately with .357 ammo (though with the snub .38s are much easier to shoot and to get quick follow up shots).
A dedicated .38 is probably going to be just a tad bit smaller and lighter than its equivilent .357. The cartridge on the .357 is longer than the .38 so the gun and the cylinder will be a little bit longer. Also, the .357 is (obviously) a much higher pressure round so you may find that the gun is strengthened over the .38 adding weight and possibly width.
If you are looking at small framed snubs (like the S&W J-frames) you may find the .357 to be too much out of it. If you are going to shoot only .38s you may as well save the size and weight and get a dedicated .38 (unless you find that the lighter .38 recoils too much for you and you want the added weight to control recoil).
For a first revolver I'd say get the .357 (and stay away from the snub). You may find yourself mainly shooting .38s out of it but you will have the option of shooting .357s when you're ready or when you want to. I made the mistake of buying a .38 first and really wished I had the choice of cartridges. Later you can add a dedicated .38 to the collection.