Depends on what you're using it for, and what you prefer.
I personally prefer a prong (single).
With the lever you only have one size (without unscrewing it) and that's a pain if you don't always want it the same tightness (I wear mine at different tightnesses for different lifts and different percentages of max).
Also if you use any powerlifting gear (briefs, suit, etc) it will be a completely different size raw vs. geared.
I have one each of these:
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The 10mm is PLENTY thick in my opinion. The first time I put it on, I honestly couldn't get it off (it was so damn thick it has not very flexible at all).