My only experience with St.Croix was borrowing a 9 foot 8 weight Imperial rod for steelhead. It was a nice rod, good action, balanced, and easy to cast. I'm guessing though you're asking about spinning rods. Here's my take on one piece rods. I've never been happy with them. I think years ago, there was a big difference between one and two piece rods but I don't notice the difference now. I have a 5 ft ultralight but even that length seems to be a pain to transport sometimes. If your thinking it will only go in a rod locker in a boat, a one piece might be fine but I prefer two piece.

I 6 foot 5 inch bass casting rod that I love! They are definitely worth the money. I have also extensively used lightweight crappie st. Croix rods that I love.
If 7" of shorter I would go with a single piece. In my experience most decent sized cars and trucks fit one just fine. Most nice two pieces will work fine however I have had 2 types of malfunctions happen with a 2 piece rod.
1- The two rod parts wont come apart
2- If they are loose enough I have had the 2nd part come apart when I cast.
Neither of these will happen with a one piece.