For expansion, if using the direct to rail clamps the rail joints will stay fixed if they are tightened down fully. In my case my curves are free floating so they can push out or pull in as needed but at least on my layout so I haven't had issues with it. (My straights are screwed down to boards (although they are more free floating in more places as time has gone on). I will note that I'm using Stainless steel which may have a lower expansion factor than brass or other metals. If someone needed expansion points in a straight, Split-Jaw does make a double long clamp that is made to allow for expansion.
Having seen both types, for me personally I would only ever use the direct to rail type. For those using over-slider joiner clamps with slider joiners and they seem to be fine that is great, but reliable track power outdoors is one of the more challenging issues people deal with in the hobby and what drives a good number to just go battery power. Aside from brass oxidation on the rail, unreliable/oxidized joints that use slider joiners are the other main source of problems and from my experience, direct to rail clamps completely eliminates any question on rail joint connection reliability. My layout outside has been deployed for 10+ years it is still 100% reliable with no track power issues. Again that is partly because of Stainless rail but without the clamps I would have issues. I started out with rail joiners on my first small 120ft oval (SS track and SS slider joiners) and within a year or two I was having connection issues. With the way those slider joiners were designed it would have been lucky if an over the rail clamp would have made any material difference given how thick/hard the slider metal was (resisted bending).
I can just say, if someone wants a truly reliable track powered G scale layout, removing all slider joiners and going with direct to rail clamps (of the same metal type) is a must. (It is good to use clamps of the same metal type ie brass clamps on brass rail or Stainless clamps on stainless rail because of Galvanic corrosion (https://en.wikipedia.org/wiki/Galvanic_corrosion)). I dont know that dissimilar metal corrosion would be a material factor for what we are doing, but the bottom line is if you are going to go through the expense and buy clamps, it is best to do it right and avoid any chance of issues.