Jorge:
I've used both. Ultrasonic can't compete with a Radar if you place a value on accuracy, repeatability, nil maintenance, and programmability.
The price of Radar units has also come down quite a bit since its inception and is due to refinements in manufacturing and the number of capable competitors.
I've used Radar on storage tanks, reactors (with internals & agitators), process vessels, and in bridles (external location). I've never had a failure in the operation and with the exception of 2 or 3 installations that had to be re-calibrated (my installation contractor mis-handled), I've had 100% success and satisfaction. This instrument has allowed me to erradicate the need for instrument techs to ever have to enter a process vessel or tank. Confined space entry for these techs has always been a safety risk. I wish I could keep everyone out of having to enter vessels - forever!
I hope this experience helps.
Art Montemayor
Spring, TX