One thing I've noticed with heads, is that it's not so much about whether will you ever need ot come close tot he rated capacity of the head, but that with the higher rated capacity heads, you get an infinitely more rigid coupling to the tripod.

If it comes down to a matter of price, always go for the most expensive model that you can afford.

Haven't checked the price differences between a Q3 and a Q10, but if it's only $100, then in the future and especially having a 70-200/2.8 already then the better candidate would be a Q10 with the higher weight rating!
The size of the ball itself if quite critical in how rigid the set up is going to be(when you need it most!)

(OK, so I got curious! )
The pricing of Markins' products seems to be similar to RRS, and having just checked, I'd say definitely go with the larger ballhead for only $60 more!!

Markins seem to be of the traditional style design, more upright and taller than some of the variants in ballhead design.

For that kind of money, you should also look at the RRS product range too. It's a lower sleeker design(which I prefer now).

It all depends on your most favoured photography genre too. If you do landscapes and some faster action stuff, like birds/sports/etc where shutter speeds are usually high, then any head will suffice.

If you think you will ever get into a photography genre where mirror slap will cause IQ issues, or you need to hang the camera out a long way out from the COG of the tripod and the slightest trace of vibration is then magnified many times over, then you will ultimately want a larger ballhead(as large as you can afford).