No, they are very different. The closest comparison to the Keeley Caverns I can think of is the EQD Dispatch Master. I had both pedals for a while. As for the Boss Tera Echo.. It's a different animal altogether. It's unique sounding for sure, but sounds too contrived rather quickly after using it for a while. It's definitely not a pedal you'd leave on all the time, if you get my meaning.
I think the Tera Echo would sound great with a synthesiser. it's not a bad pedal at all. It's unique sounding and to be honest, I can't think of another pedal that sounds like it, but much like a 'shimmer' effect on some reverbs, it's the type of effect you don't want to overuse.