I believe you can go down to TKfoto and see both holders in person
Both fit the 14-24 very well, and both hold the filters just fine.
I chose the Lee SW150 over the Progrey because of 3 things:
1. The size of the adaptor rings
Lee's adaptor ring is smaller and can be left on the 14-24 at all times, while still allowing the lens to be stored in most bags. Progrey's adaptor ring on the other hand is quite physically large, and if you leave it on the lens all the time it can be hard to fit into some bags. Both the Lee and Progrey's filter holder sections can be removed easily.
2. Ability to use on other "normal" lenses
Lee makes an adaptor for that purpose, Progrey does not.
3. Ability to seal against light leaks
This is only really relevant if you use high strength ND filters in the day. Neither holder is perfect out of the box, and I would say that Lee's holder does have some interesting design flaws. But personally, I can seal the Lee holder better and more easily than the Progrey. You can PM me for details about this.