For me personally is the very best choice The!Cart. It has the best menu and it is by far the fastest. The contents of my cart do not change many times. I have everything on it I want/need and once in a while (approx. once a year) I add something to it.
I have all the other multi carts here too.
I also am a huge fan of atarimax MyIDE ][
Uno cart is nice, but way too limited. Ultimate SD cart is great, but I have stability issues on some of my atari's and I do not appreciate the loading time on larger roms.
I have my The!Cart permanently inserted in my atari. The very first entry is my SpartaDosX rom. Since The!Cart loads everything instantly, this is a pretty cool substitution of my SDX cart. When I want to play a game I pick something else from the menu. I even have a 'boot from Sio' menu entry created. The menu on The!Cart is the best menu i have ever seen. Simply type a part of the title and boom there it is. You don't even have to type the start characters. When I type 'Man' on my menu I get this:
Enigmatic Man (msx demo)
the menu results change real time! It is incredible.
Another superb feature of The!Cart is that it does load certain ATR games that do not run on the other mentioned carts. I have Cropky on my The!Cart and I also have Johny's Trouble on The!Cart. Both the DISK versions.
Since you already have Sio2SD it is a piece of cake for you toe program The!Cart.
That is the only downside of The!Cart, you have to flash it. And when you are starting with creating a nice collection, this could take a while.