I debated on responding since I don't actually own any of the three models you mentioned...as you specified in the question. However, I have some thoughts and experiences that I'll share.
I've used a Surfmaster (not DF) a lot. White's been making the series for many years and the physical configuration has been time-tested...they simply don't leak. I know several folks who hunt the DF model and they all swear by them from a depth perspective. The DF warranty is only two years.
After looking at the specs, the Garrett seems to be the least attractive of the three. It only has a one year warranty. Fixed sensitivity (vice adjustable in the other two) is a definite drawback IMHO. I'm also a fan of hard-wired coils and headsets...vice the waterproof connectors of the Garrett. The Garret is also over a pound heavier that the other two machines.
The Sand Shark is a very popular machine. The Lifetime Warranty that is a mainstay of all Tesoro machines is a definite plus IMHO. The Voltage Controlled Oscillator (VCO) function is a nice addition to a PI machine. Of the three, the TS is the only one with adjustable volume...again, a nice feature.
If I were to line them up in order of preference, I'd pick...
1. Sand Shark
2. Surfmaster DF
3. Garrett Sea Hunter (distant third)
BTW: It may seem odd that a guy in Nevada is making comments about beach machines.

We have a second home in Florida (6 miles from the Atlantic) and I've spent a lot of time in the surf off Santa Cruz, California. I just moved from California about eighteen months ago.