I switched from an 18 to an VX1 (step down from the M3/lx3/etc) which far outperformed the 18 in almost every fishing situation, As i sold my 18 to my fishing buddy to upgrade his FL8 and fished them side by side for years until he upgraded to an lx5.
A few people have mentioned low power mode of the fl18. This is a completely necessary feature of any Vexilar because when you turn your gain dial to 0, it does not clear the screen. you can still see you jig (in less than 15' of water) you can still see the bottom. So in shallow water situations, your tiny jig will appear to be a 1' big mark on the flasher. low power mode dials that down to 6" but that still a lot of mark for such a tiny jig.
The Marcum completely clears the screen and then you can turn it up to 1-2 to begin to see the bottom and then a smidge more until you see your jig as a thing orange line. with the Marcum low power mode is your gain dial.
In 15-30' of water both of those units are absolutely dynamite. In shallow water, the marcum will shine.