Full disclosure, I am an unabashed Quorum fanboy, so my apologies to the Unitrends folks 
The short short version of this post: Go look at each of the respective appliances at the price point you’re looking at and look at their raw hardware specs (CPU, RAM, Disk and Disk Types (SSD vs HDD ratio)). Ask yourself which one would make a better, more capable host to my virtual machines. That will also tell you where each company has put their focus in regards to backup and DR.
I’ve been a Quorum customer for almost 4 years and it has been an awesome experience with absolutely no regrets. We hadn’t even heard about Quorum until we saw a demo at a seminar about a week before we signed with Unitrends (which we had thoroughly evaluated up until that point).
Like the OP, our key requirement for a DR appliance was to get back up and running quickly, and we thought (up until we saw Quorum) that Unitrends was the only appliance based solution that could fit the bill. After seeing the demo, however we knew that Quorum was far more closely aligned with what we were looking for.
In a (much) earlier post I detailed the key differences between the two systems: Click Here
Here’s the TL:DR version comparing the two systems
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The core focus of the Quorum box is to build a standby clone of your server (physical and virtual alike) that is kept up to date and automatically replicated offsite. It can also do the usual recovery tasks of File/Folder/BMR restores
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Unitrends was great at ingesting backup data, but for a full site recovery it required a lot of extra spare hardware to recover back to
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When you protect a server with Quorum it automatically does everything necessary to build the VM. It builds the VM and replicates it offsite by default. You can change this setting for less critical servers, and build the VM at a later date if you need it.
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When we last looked at Unitrends, you had to designate the servers to be “Instantly Recoverable” ahead of time. and it was not done by default. If you didn’t do it prior to a disaster/sever failure you couldn’t do it later. Furthermore you had to allocate additional space on the Unitrends box for the VM to reside. If this has changed in later versions then I stand corrected.
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The whole Quorum box is essentially a “Hyperconverged” backup appliance. It runs Citrix XenServer as its hypervisor
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I don’t know what Unitrends uses for it’s virtualization platform, but I’m pretty confident its not running a full hypervisor
The tipping point for us was when we asked both our Unitrends rep and our Quorum Rep the exact same question: “How many virtual machines can I run on this appliance?”
Our quorum rep gave us a straightforward answer, the OnQ 288 (the appliance we were looking at), will support about 40 VMs
Our Unitrends rep, and SE couldn’t answer the question directly. The gave so many caveats and qualifiers the best we got out of them was “maybe 5 or 6”
We have done 4 full scale DR tests with our Quorm boxes (we set a goal of a 2 hour RTO and we’ve beat it every time), it stepped in and ran my email server for 45 days when the old server died. I have seen the tech perform admirably in the world and I have complete confidence in it’s ability to perform.