I'd love to respond to this question sincerely, without the emotions of some here ruining an otherwise educational opinionated discussion.
First, my experience with each:
I owned Duende for a period of about 5 months, when it was first released. This was the full rack mountable version, connected to the computer via Firewire.
I have owned a single UAD-1 card since 2001, when it was first released.
So for about half a year, I had both on my system at the same time. And I was mixing a lot in that time period, so i had a chance to really dive into it.
The first problems came almost immediately. Using UAD plug-ins at the same time as Duende Plug-ins resulted in audio drop outs, severe clicks and pops, and in some cases, crashes.
I tried installing a PCI FW card, switching PCI slots, etc... nothing worked. Duende's tech support tried to help, but wasn't able to correct this problem. It turned out to be a combination of my G5's limited architecture, coupled with Duende's early drivers, which quite frankly, were not written very well, imo.
I was able to get work finished by always freezing tracks that used UAD plug-ins. This way, I was able to finish a mix using both. But as you can imagine, after a while, this really started to become a cumbersome way to mix, and ultimately, I decided it just wasn't worth the extra effort.
Because I had invested so much in UAD's plug-ins at that point, coupled with the fact that until I bought Duende, I had had a very stable system, I opted to let Duende go.
Regardless of my technical experiences with Duende's early outings, here are MY opinions...
Duende has that SSL sound down. the emulations are very, very good. the channel strip's EQ is among the best I have ever used. It's compressor sounds like an SSL channel compressor... which would be it's only negative. It has "that" sound. That can be a desirable feature, or not... still.. having SSL type compression available on a track doesn't suck... 🙂
Duende's 2 buss compressor sounded equally as good. Slap that on your master out, and sit back and smile...
I did not, unfortunately, have Duende long enough to get to use their newer plug-in, the ones optimized for drums and vocals.
As I stated, I became a UAD-1 user when that card was first released... back when I was using a 400 Mhz G4. I owned that same card up through my G5, and just recently sold it, as I moved to a Mac Pro, which uses PCIe slots. I replaced it with a UAD-2 Quad card.
The UAD plug-ins are ALL amazing. There truly is not a dog in the bunch. The compressors are worth the price of admission alone. that being said, I've never been a fan of slapping 1176's or LA-2A's on every track. These compressors have loads character... a lot of it. placing them on every track diminishes their uniqueness.
The EQ's available for the UAD card are also stunning. The Neve's are the bad boys here. The Neve channel strip can do what the SSL channel strip can do... similar functionality, in that there is a parametric EQ, compression, and gating, all in one plug-in. But there are also some cool optional EQ's , like the Helios, and the Pultec, which is truly an amazing sounding EQ. Limited in design, yes, but the sound more than makes up for that. It's not a surgical EQ... the Cambridge is better suited for that type of functionality.
Here's my opinion... both Duende and UAD will add a wonderful dimension to your mixes, providing you know how to use these tools. They both sound great... they really do.
One is not better than the other... they both can serve the exact same purpose. I was finding I preferred Duende's EQ on some things, but not others. I had options, which was wonderful.
Long term, I do believe that even though it's more of an investment to go with a UAD card (once you start buying all the optional plug-ins, and there's a LOT there), it's a sound investment. If you think you might want more than just great EQ and compression, it makes sense to take a closer look...
Whatever you decide to go with, you'll be getting some of the higher quality plug-ins available. It's a win-win situation, as I see it.