Yeah I would agree. Without doubt, Marillion took a lot of influence from Genesis, and Fish from Gabriel, but if anything I feel he was more a student of Peter Hammill than Gabriel; the latter tends, as you pointed out, mostly to croon and sing relatively softly (so much so that when he spits and growls on tracks like "The Knife" and a little in "Stagnation" it comes as something of a shock) whereas Hammill could go from angelic choirboy to raving devil in an instant, and back. He could be completely sane and then go off on a madman's rant, and return to normality as if nothing had happened.
The music itself drew much from Genesis, but it's very different. Look at the debut: the first song is a lament for a lost love, the second a sharp indictment of drug addiction, the third a retreat from life itself. Follow that up with one satirical upbeat song about the "posh folk", then right back down into one about unfulfilled dreams and suicide, and end with a rage against the Troubles in Northern Ireland, seen from a British perspective (duh) - doesn't sound like any Genesis I know.
If any neo-prog band ripped off Genesis I'd say it's Pendragon. I mean, I love the guys to death but they literally took sections of "Firth of Fifth" and I think "Cinema Show" and shamelessly used them in their songs. They also ripped off Supertramp and Floyd, but that's another matter.
I think the problem with Marillion is that they were always going to be compared to Genesis, being one of the first bands to bring back prog into the mainstream, and one of the best known and biggest survivors of a trend which left a lot of bodies in its wake. A lot of people could not see past the Genesis tag, but I think it does them a disservice. Genesis were always more esoteric in their lyrical matter; Marillion went full-on into politics, human relations and controversial subjects. Marillion were definitely not the 80s version of 70s Genesis, not by a long way.
You can read more of my rantings about Marillion
here