Skagits are great for chubby flies and sink tips. When it comes to cold water fishing I like to switch to a skagit just to help to get the fly down in the water.
"are there otherlines the same or not" skagits are all about the same no matter who makes them. If you talking about Scandi, Skagit, mid or long belly then they are fair apart in just about evey aspect. On the GRonde I like running a scandi head with a poly leader for skating bugs but change to a skagit around Dec/Jan depending on the years temp.
I resisted using a Skagit head for a while, but after dealing with longer belly lines while my ass was backed up against the bank and standing under over-hanging alders, I relented. Skagit makes difficult situations easier.
Think of a skagit line as a diesel-powered truck: it can pick up and haul almost any load. You could do all your spey fishing with a skagit system; the power is certainly there. But you'll probably fish smaller flies, and where you need less sink, for some of the year. So scandi and mid-length spey lines do that with more ease and grace.
I much prefer casting a mid belly line....
...until you're on-stream with your butt in the brush, or you want to throw a brick of a fly. The down side is they don't take much talent to throw decently so it's not a spey technique improver. The up side is they don't take much talent to throw decently so you're in the game pretty much anywhere.