I would put the bullfrog in the 55 gallon. I live in Illinois and we have an abundance of large bullfrogs, and I can tell you from experience, these guys can get 8 to 10 inches long from nose to toes, and can use every bit of space that you give them. Also, being a fairly aquatic frog, bullfrogs love plenty of space to swim, and half to three quarters of your enclosure should be dedicated to a swimming area. You can pile gravel at one end of the tank and do it that way, or use a divider to create a separate land area. Just make sure that the frog can easily access the land area. Your local pet stores should sell something called a turtle dock (I think Zoo Med makes them), they work well also. Another idea if you are creative and even slightly handy is to silicone large pieces of cork bark or driftwood to the back glass, effectively making a land area above the water, just make sure to put a piece on an angle sticking into the water so that the frog can easily climb onto it. Also make sure that if you silicone a land area to the glass that it is well supported, bullfrogs can get pretty heavy. You can silicone a piece of driftwood under the cork bark leading down to bottom of the aquarium like a support column, that works pretty well and looks cool, too. You will also want to use a filter for the water, as bullfrogs will soil the water pretty quickly (they poop in there). There are several brands of in-tank filters at your local tropical fish store that will work perfectly for this type of application. Lastly make sure that you have a very secure lid. As you said yourself, these frogs are excellent jumpers, and with them being the largest frogs in North America, they have the ability to knock traditional fish tank lids right off of the aquarium, and the last thing you want is a large bullfrog hopping through your house, or drying up and dying somewhere. I hope some of these ideas work for you, or maybe spark some other ideas of your own. The best thing to do is have fun with it, the options are limitless.