I asked this exact question from a friend, Craig at Red’s Flyshop, last week. His response was it depends on what type/size fly do you want to use. It sounds like you already have the set up for fishing streamers and indicators, so if you are going to use it mostly for soft hackles, as you stated in the post, I would go with the single handed spey. Keep in mind that it will handle like a spey line, so if you are not proficient casting longer lines on the two handed rod, you will likely not like it, at first. I don’t own either of the two lines that you mentioned, but will say that although I have more heads and separate running lines than I have integrated lines, I much prefer fishing the integrated lines.
The nice thing about the single handed spey and fishing soft hackles, you won’t need to waste any time stripping line in, just cast, swing, repeat...until interrupted by some pesky fish that you have to release. I don’t think the single handed spey line would be much fun fishing with indicators.
I saw an impressive demo with a Sage one 4 wt. and this line and later I tried the combo at Red’s Rendezvous a couple of years ago and it is one of those things that haunts me to this day...it was an amazing combination but I just can’t see spending that much for a rod /line combo for trout.