That's a hard question.
If you already have shallow and deep 3/8" sockets, I would say go with the swivels.
Both honestly, you're going to need (or at least want) both at some point in time. Depending on what you're doing, that's the reality.
The swivels are sometimes the only way to get a bolt/nut off or on. Putting a socket onto an swivel or universal adapter or a wobble extension will be just too big, even if you use a stubby socket... I absolutely hate universal sockets, but I had to buy a set after a 10mm 1/4" universal literally was the only way I could reassemble a transmission filter housing. I struggled for what seemed like hours, but thankfully I had that damn socket. I tried every way to Sunday with no luck, except that socket... You'll likely have a similar experience at some point with swivels.
I have shallow, mid-length and deep 3/8" sockets, and a set of 3/8" swivels
You don't have to buy Snap On, but this is the set I have, minus the 8mm (which I still need to buy)
shop.snapon.com
I also have the smaller 1/4" set, but I need to pick up some more individuals to fill in the size gaps
shop.snapon.com