Both are good two lane roads. Have taken both of them with the MH. For me it would depend where I wanted to go on I-5. If I was going anywhere from Eugene North I would take 126 if I were going from Sutherlin South it would be 38 to 138 to 5. In between it is a toss up.
If you go over 38 and want a good meal you can stop at my brother-in-laws place in Elkton (Tomaselli's Pastry Mill) tell him Dave and Jeri sent you. No price break but he will give you a good meal and a bad time