i can only find commercial descriptions of both.

Sorry no one has replied to your post. I have no experience with either because, as you have discovered, they are both commercial grower varieties grown for shipping/market. That alone is enough to make most home gardeners shy away from them. Why grow what you can buy in the store? is the philosophy.

If you do try them be sure to report back on the results.

Dave

yeah its true ultrasonic is a commercial variety,i don't know about monte carlo because its indeterminate and sold as a home garden variety by italian seed companies (franchi and bavichi) i've had pretty good luck with some commercial varieties , one hybrid called primetime from harris (no longer available ) two open pollinated commercial (or once commercial) are campbells 1327and basket vee from stokes, both pretty tasty and good producers. i've never tried the original marglobe but i hear its worth growing, have you grown it?

I have grown Ultrasonic Tomatoes. They were one of nicest looking and excellent flavored varieties I have grown. Certainly would recommend.