The 6000 Di Pro SL has the volume control, like you pointed out. This is not a big deal since almost all aftermarket headphones include a volume control on them. I just use the volume control on my Jolly Rogers.
I have both the Di Pro SL and the XL Pro, and I think I like the XL Pro a bit better. Of course there is the size and weight advantage to the XL Pro, but besides that, the XL Pro is just a tad smoother and deeper. It's close though, and might just be placebo effect but most guys that use them both seem to agree the XL Pro is just a tad better in performance. The way I understand it, White's took the circuit design of the old 6000 and started with a clean slate, using the best components possible and tweaking it for low noise, when they redesigned the XL Pro. That makes sense, because the XL Pro does seem a little quieter and falses a little less.
One thing to think about is that if you keep the XL Pro and get then MXT, then your two detectors will share battery packs.
I would keep the XL Pro, sell off the Di Pro SL, and use the money to put towards an MXT. You can find very nice original model MXT's for around $400 used, so you'd only have to put in $100-200 more after you sell your Di Pro SL to be able to buy a good used MXT. The MXT Pro is not worth the extra expense, the non-pro MXT is just as good performance-wise. Everybody seems to want to Pro, so the non-pro MXT's are usually very good bargains. I can live without the additional features of the Pro which don't do anything for performance anyway, they are just bells and whistles.