I'm considering buying a Ruger Blackhawk in 357/9mm or a super Blackhawk in 44 mag.  Both would be 4.6 inch barrel.  I would only be shooting low power stuff like cowboy action loads, but full power might be nice for special occasions.  What's your opinions on this?

All useable calibers in a handgun in consideration of your stipulated criteria start with the number 4....

We could use a little more more information than just you intend to shoot light load and might use full power later.   What are you planning on using it for?  What might you use it for in the future.

They make the Blackhawk convertible in 45 Colt with a 45 ACP conversion cylinder too that you might consider.

The 357 Mag version is going to be the cheapest to feed.  Super cheap if you're a reloader

The 44 Mag is going to be the easier to buy powerful ammunition for if you want it.

Either the 44 Mag or the 45 Colt can be hand loaded to fairly impressive power if desired.

The regular Blackhawk will be easier to carry on the belt due to its weight and size compared to the Super Blackhawk.

The 357/9mm gives you more ammo choices.