18 ft fiberglass will usually be more expensive, ride better, be less effected by wind and waves
17 ft (would go at least 60" bottom) will be more affordable, a little more forgiving in brush and rocks, easier to tow, and require a smaller tm and less batteries. Fish out of each before you make a decision. I have a 20 ft aluminum 8 ft wide with a pad hull much like a fiberglass. It rides tons better than the mod v, but still not as good as glass. For my style of fishing it is the best of both worlds.