Ok - I just went and grabbed both. I have a BM550/PE/Sheepsfoot/Thumbhole and my Kershaw Whirlwind, which is PE. I have med. small hands (cadet, but golf terms, which means slightly long fingers). The Griptillian has these pluses for me:
- Feels better in hand, hence will work better when a forceful downward cut is required
- Clip positioning - I can go right/left/tip up/tip down. I'm a very strong lefty.
- Thumbhole - Not as good as Spydie, but very manageable.
- Sheepsfoot shape - Surprisingly useful in a knife this size.
- Handle patterning - slightly better than the Whirlwind, should be better in wet/slippery conditions - only spec here - haven't done a comparison.
- Axis Lock - being a strong lefty has a huge advantgage for me here.
It has very few downsides but it does have one or two:
- Partial steel liners give it a bit of a cheap sound. I don't doubt it would more than hold up but.... In a knife of this size, the extra weight that the liners would have imposed, may actually have been desireable for me.
- Thumbhole should not have been chamfered. I have very dry hands and my thumb does slip out occasionally when I try to deploy quickly - something my Spydies don't do
The Kershaw Whirlwind has these things going for it:
- Assisted opening - It actually works very well.
- The knife itself, despite being every so slightly thinner does fit my hand very well. Just not as good as the Griptillian.
- The bladeshape is to me, sort of a cross between a drop point and a clip point. Probably closer to a drop point, but a useful shape nonetheless.
- I haven't used this one all that much, but the linerlock hasn't budged, screws haven't loosened up since I got it. I've had a few where pivots have loosenend and this is definitely not one of those.
Here are the things that sort of keep it on the shelf:
- Only one position for the clip - tip down/right.
- thumbstud is not reversible - not that I can't manage, but.......
- Because of the AO, it's a little harder to close it than the BM
- No lanyard hole per se. I wouldn't say this is that big of a drawback, just a preference.
- I think it's because the handle is slight more rounded on the Whirlwind, but when the AO flicks the blade open, it makes a very hollow cheap sound. Same when you close it.
They are both in roughly the same price point up here, the 550 being slightly more. I haven't done that much cutting with either so I'm not going to venture an opinion on edge retention. For the things that I do, either will work fine. This is just my thoughts on it. I have several friends who have Whirlwinds and absolutely love'em. I don't mind it, just prefer the Grip. I would suggest picking up both if you can before deciding. Both should serve you well.
I collect at a pretty voracious rate (I have a pretty good "allowance"

), probably several folders a month. Sometimes I go into withdrawal, and can't find something I like, so to get by, I will buy something non-knife. Last week, it was a small digital camera for my Maxpedition - A Fuji A400, but that was because I couldnt' get to Target Knives to pick up my Endura 4's

I needed a tax deduction against my rental properties, yeah, that was it........
One of the fun things for me in my collecting is to try to find replacements for my current EDC's. I do cut things several times a day, so it doesn't take long for me to determine what I do and don't like. I also wear cargo pants on a regular basis, so it allows me to carry several blades, actually a LOT of blades

I will usually carry a new acquistion in one of my cargo pants pockets, but will reach for it all time for a day or two just to see how it works. If it works better than what I have in my front pockets, something will have to go. Now interestlingly,in the last 5 months or so, the Spydie Delica 4/PE/FRN and a recently acquired Vic Soldeir were the only blades to make it to my front pockets. The Delica 4 replaced my Delica 2.
My usual daily carry:
- Left front clipped - BM556, Delica 4 and SAK Recruit loose. Soldier has found it's way in there the last few days. An SE Delica 4 is also being rotated there, on occasion.
- Right front clipped - Spydie Meerkat, Dragonfly loose.
- IWB Kabar/Dozier Folding Hunter.
When I wear cargos, I have a small piece of leather (2 or 3 oz) that I cut out to fit the pocket and will rotate new ones there.
Lately - left cargo is holding my Delica 4 and a Buck Northshore - has been known to also carry a Grey Calpyso Jr. and/or Centofante 4. Right cargo - Delica 3 and a Centofante 3 and/or a Camillus Blaze. I'm still deciding about the Northshore - it's a gorgeous folder and has so far worked really well, but I doubt it will make to the front pockets. Definitely a suit knife, though
Hope this helps - gord