In my opinion, either one would be a nice upgrade. If you look at the audio output power, both are over 1 watt. The audio output of the CLS is not stated by Motorola which leads me to believe that it is far less than 1 watt.
Audio output power could be considered much like a stereo amplifier - more power is better. Higher power leads to better readability, even at lower volume levels. They also use better speakers and are more weather resistant.
As for which one, they are designed for different purposes. The RDU operates on business frequencies that require a business licence to operate in most countries, while the DTR radios operate on unlicenced frequencies. The RDU is an analog radio, while the DTR is pure digital.
As for range, UHF will be better in buildings and urban areas anyway, so both would be suitable for your purposes.
My experience with the DTR radios is that they would easily carry 1 to 2 kilometers outdoors, depending on terrain.
The advantage of digital radios is that they are very clear regardless of range. The disadvantage is that once they are out of range, they do not transmit at all. (Before they open the channel, the radio sends out a short “handshake” signal to ensure there is at least one other radio on your channel in range.)
I would suggest that if you have a business licence and an assigned frequency, you will be happy with the RDU radios. If you don’t, then the DTRs are a great way to go.
The DTR radios have almost 1000 “channels” - which are not actual channels, but channel-hopping algorithms - to choose from, so even if they use unlicenced frequencies, they are very secure. There has been no documented instances of anyone being able to monitor a conversation on them using any kind of consumer or hobbyist scanner.