Hey NickyBass,
the Ehrlund is very different to the Underwood in sound and decay. Very nice to the more traditional jazz styles, but if you want to try more modern stuff like rock, funk etc. that's not what it does well soundwise. It might work with volume, but could also feedback. The strange thing with the Ehrlund was that I got feedback at home, but not on stage.
My old ATM35 is really feedback resistant compared to other studio condensers, but less feedback resistant than the Ehrlund. And As I read on Talkbass, the DPA sounds better than the ATM35 but is less feedback resistant.
The feedback resistance of the Underwood can be much higher than the Ehrlund, but it depends on pressure on the Underwood and position of the Ehrlund. But in general the Underwood is much more feedback resistant. With the Underwood feedback often comes over the open strings, so you need to damp them to avoid feedback in loud volume situations.
I haven't had an Underwood myself, but a Shadow SH-950 which is basically a clone of the Underwood.
You should know that finding a good position for the Ehrlund is not that easy. There are good hints in the Ehrlund threads (better read the completely, you waste more time if you don't read them). Finding a good position (soundwise) is easier for a close mic, because the range of the top that gets amplified is much larger than the spot position of a contact mic like the Ehrlund.
Keeping the Underwood as a backup is a good idea, I keep a Full Circle and my old SH-950 as a backup and for more modern sounds.