I would buy a SDC to compliment any deep seeking PI, not just a GPX, if you have the spare cash. In my opinion they would be great used in conjunction with anything from SD series through to GPX series & in many ways could make it unnecessary to upgrade from SD/GP series to GPX especially if your finding gold with & happy with your older PI anyway.
When the SDC first came out I was thinking it would be a great machine just for mopping up after the GPX's have been over areas, which it is, but now realise that the opposite can also be true I.e. after going over an area with the SDC using a SD/GP/GPX to then clean up deeper bits, if any, is also a viable option, in my opinion anyway, having used the SDC as well as a SD2200D & GPX5000. In fact having sold my GPX & not having the funds to get another I am on the lookout for a GP3500 or similar for this reason if one comes up at the right price. Permanently attached to a 14-16" coil will see me cover a lot, if not all, local scenarios as well as other areas too.
I have noticed some experienced &/or fulltime prospectors that share similar thoughts to mine for detecting in a lot of areas - they are now using the SDC first & then following up with their GPX etc. not the other way around, so rather than "mopping" up they are seeking out runs or patches with the SDC first.
My gold finds with the SDC, using it in my area, are more frequent & would go close to outweighing my GPX finds in a much shorter time period & for that reason it will remain my first choice detector regardless of what other machine I get including if I had another GPX!
NOTE: if I was permanently based in WA or another deep nugget location my machine & opinion would no doubt be different. Location, depth & size of gold in your main detecting area need to be considered. If your in an area of mostly small, shallow gold its a no brainer to me.