Tripod head with geared adjustments.
Nothing in a geared head in that price range, but one thing I have used in the past is a slow motion control, such as the [urlhttp://www.opticsplanet.com/vixen-piggyback-1-4-20-fine-adjustment-unit-dx.html]Vixen Fine Adjustmen[/url]t. These give you a nice range of fine adjustment, but this style will set the scope up off the head a bit and rob you of a little stability, but certainly a more affordable option. Have also seen some fine adjustment styles that are a bit lower profile - they compress a spring, though that style is a bit hard to find.
The XDB leg set has a simpler design center column, more traditional t locks for the legs and no leg warmers. The XB gets you faster lever locks, leg warmers and a slightly improved design on the center column. However, none of this will have any effect on the head you use and, in fact, I prefer the older t-locks to lever locks. The t-locks are slower to set up, but they are more reliable in mud and sand, dirt and so on.
It is certainly a good choice, yes, in terms of stability, but in terms of weight, the Super Pro is probably more weight than you want to carry if you plan to throw the tripod/scope over your shoulder and carry. Do you really need a geared head? For general work, a good smooth, liquid head, like the 701 or 128 will do fine.
We only sell the 701, now, as part of a kit, but just go for the 128. Really no advantage of the 701 for spotting scope use. Has an adjustment plate for balance.