I've had both the Obscura and the Polara to use on synths.
I've still got the Obscura and I quite like it as a basic delay. I've usually got it on Tape or Lo-fi settings and it sounds pretty good for the price.
It can get quite dirty sounding.
I had the Polara for a few months but never quite bonded with it. Not quite sure what it was but there was something in the tone that didn't agree with me.
I ended up selling it and putting the money towards an Eventide H9 - much happier now.

Reverb really is personal taste though. A lot of people seem to like the Polara.
Both pedals seem to handle line level no problem. I've used both with synths plugged in directly and on the sends of my mixer.