I don't own either rig, nor do I have any experience with either outside of what I've read about them on various forums.
Seems like the D3 is the slimmer of the two, while the UW has a bit more storage space on either end of the rig. They both have GP pouches, two pistol magazine pouches, the D3 has the "stuffit" pouch to hold all sorts of random items, each holds four rifle magazines, both can use the swiftclip system.
D3 is around $180. Mayflower is around $200.
Personally, I like the Mayflower more only because it has the extra space for a radio and an IFAK/whatever. The Mayflower also seems to have a bit more in the way of comm/hydro routing loops.
To me it seems like the D3 was really meant to integrate with a PC where you'd already have space for those extra items, while the Mayflower is more a standalone system.
My question to you would be: what do you need to carry and how well does each rig fit those needs? Obviously, they are both good, well made rigs, so you'll be getting a good product whichever one you select.
Lastly, have you looked at the Skdtac copy (aka PIG UCR) of the old Eagle UCR?
It holds 4 pistol magazines, 8 rifle magazines and has MOLLE space to add a radio pouch or GP.
I like playing more lightweight but I also planned on using the rig as a standalone kit so this is where I began having indecision - the D3 is much smaller but the UW Gen IV has much more to offer in terms of item space. I also planned on using it for rec. shooting (a KAC SR-15, specifically) and I don't have a PC to attach either to. I've never been a PC kind of person.
I actually haven't looked at the UCR but after taking about fifteen minutes to research it I'm beginning to lean more on the UW GEN IV - it has the pouches I want readily available and it sits on an H-harness (which I prefer over crossback straps) without having to be converted like the UCR does. For the price though it looks like great kit, and I trust PIG's quality (I own their FDT gloves and they're some of the best gloves I've ever owned).
I'm ony phone, so I'll keep this short. I had the D3 for about a month before I sold it. I really just wasn't that impressed by it. The mag insert that was farthest to the right was almost inaccessible with a pistol in the pouch, the shoulder straps constantly slid off, and it rode really low. That's the bad things about it. If you want to know the good, watch any review on it. It is a decent rig, but a lot of people are considering it to be the holy grail of rigs because TRAVIS HALEY OMG. Its a nice setup, but built more for rapid deployment/active shooter or a day at the range. Not airsoft.
The Archangel Gabriel will sound Judgement with a blast from his trumpet. I kind of do the same thing with my rifle.
Yeah, that's the impression I get when looking at the features on either the D3 and the UW. More for a day at the range than for airsoft. They both seem to skimp on magazine pouches while providing a lot of extra storage in GP pouches, the "stuffit" pouch, etc.
Obviously first responders, law enforcement and military folks have other pieces of kit they need to carry to complete their tasks, so I totally understand the need for more GP type storage. For airsofters, all we really need to carry are primary and secondary magazines, speedloaders, loose ammo, a radio, maybe an extra battery, flash light or tool.
As I mentioned above though, if you must have either the D3 or UW, they're both made by reputable companies so they should serve you well for a longer time.