Recommendations will be very subjective. I like and fish exclusively with nylon monofilament running lines.
As a personal preference, I err on the side of thicker, heavier pound test. It is easier to handle when the air temperatures are cold or one is tired. Thicker running line is also less likely to slice up your hand. Note that I usually fish in close. If making beautiful hero casts is a necessary part of your angling experience, then by all means go for smaller diameter, lower pound test running lines.
I have used and like Guideline Compline, Ken Sawada (no longer available) and the OPST Pure Skagit Lazar Line.
For your 5 weight switch rod, I would recommend looking at the OPST Lazar Line
in 35 lb. or 40 lb. test. I rarely if ever have to stretch the Lazar Line. Get it a little wet and any coils straighten out very quickly.
It is expensive but should last for years. If one end gets beaten up, simply reverse the line.
One more point. I use these running lines for Skagit heads with heavy, fast-sinking tips as well as floating heads that are used almost exclusively with dry flies, for trout, for example. These short 2-handers are great for dry fly fishing. Just amazing. The trick is to stop thinking like a 'swing robot'.