Depends on the line length (and how many lines you have), how much wind you have and what kind of flying you're doing. Oh yeah, and how big your kite is and what kind of kite it is.
So, it depends.
Assuming we're talking SUL kites in light wind conditions, not in zero wind. That's different.
With duallies I find that on full size kites I can't tell the difference between 90# & 50# at 50' and barely any difference at 75'. At 100'+ the weight and drag of the heavier line is more obvious, but I still don't bother with 50# unless I'm trying to pull off a zero wind ballet and need all the help I can get.
On smaller kites the line weight is more noticeable, but then I'd rarely fly a small SUL on lines longer than 75'.
I tend to fly older kites with lots of lift and drive. Modern SULs are by necessity less efficient, so they need all the help they can get to stay in the air.
This is a very easy question to answer for yourself (your conditions, your kite, your style) if you have both line sets.