Wetton is a fine bass player, but his playing with Heep was a bit heartless. Hensley once said: "Either very standard or completely off the wall, nothing in between!" Thain had more of an intuitive feel for the music (of which he wrote very little), he played complexly, yet it sounded natural and even simple.
Wasn't Heep's major exposure to the US market opening for T. Rex in the early seventies? Not that T. Rex cracked the US market wide open either. Heep are not even a Euro thing, but more of a Germano one. Not even in Britain did they ever have the success they had in Germany. Derided as "poor man's Deep Purple", they still did quite well in a market where Deep Purle meant all the world in the first half of the seventies. Heep weren't as popular in Germany as DP or Led Zep, but close and certainly more popular than, say, Black Sabbath, Mott the Hoople, UFO, Nazareth, Thin Lizzy etc.
The production of those early Bowie albums did Trevor Bolder no favors whatsoever. Strange, given that Tony Visconti is a bassist himself.