18 is smaller than 24, so you actually get more of it on a given sppol or reel. I use 18 on some of my reels. It is not as strong as 24, but the difference is minor and it is strong enough for the intended purpose.
With any line, abrasion will reduce the tensile strenght of the line and frequent replacment is a good idea if you see the line getting abraded. The initial 50-75 ft of the line sees the most use, so before it gets worn to a significant degree, you can reverse it on your reel and essentially double the useful life of the line. Similarly, if the first 20' gets really worn, just clip it off and tie a new loop in the end, you won't notice the difference between a 400' reel and a 380' reel.
On the other hand, line is one of the cheapest things in cave diving, so I just replace it when it starts to show a little more use than I am comfortable with.
I am not a fan of Dive Rite reels as they do not allow the same degree of control over the line tension as a Larry Green or Halcyon reel and that is the best means of preventing a birdnest, but if you get one, definitely get a side handle reel.
edit:
I am not sure about the actual diameter of #18, but the rated tensile strenght is normally 160 pounds. In comparision:
#24 .073" in diameter and 230 pound tensile strenght
#36 .083" in diameter and 330 pound tensile strenght
#48 .098" in diameter and 405 pound tensile strenght
1/8" .125" in diameter and 728 pound tensile strenght
Again the issue is not so much line strenght, but abrasion resistance. Line that is properly tied off in a cave is not going to be subject to much abrasion compared to line runnng through a wreck where current or surge may be moving fairly sharp pieces of steel marine growth back and forth against the line.
