Hi Doug,
   I have a gtx330 in my cessna and a cheesy portable xm weather setup there too.
   I have an mx20 and wsi in the lancair IV but it is not flying yet.
   Comments below.

gtx330 traffic
   really worth it when you are in a coverage area but lags a little and sometimes reports yourself which can be a real scare.
   I personally don't have confidence either that mode s traffic will be systematically dismantled soon nor that ads-b will be systematically installed soon.   For now, there is more mode s coverage so I would say spend your extra $3000 and consider it rent for 5 years service.
   That said, the most useful thing about the gtx330 is that it convinces you that you really want an active traffic unit like the ryan 9900bx or skywatch.  The former is cheaply packaged these days.

mx20
   really pretty and pretty ridiculous for weather download IMO.  You only get a 250 mile diameter and panning is slow.

mx20 and wsi
   mx20 is not supporting any new wsi features (winds aloft, graphic pireps etc.) and that's a pain.  However, wsi does allow you to split the data feed and run one side to a pc.  With the pc you get all wsi features, can see the whole country at once, pan with speed.   If I had it to do over gain I would ditch the mx20 and the av200 and buy a av100 with a es800 pc instead. (avxxx is the wsi receiver).  That would have saved about $10,000.

mx20 and gdl69
   supports new features but still has the 250 mile limit and is slow

wsi vs. xm
   wsi pricing is nuts.  However, they cover Canada and xm does not....so far. (as far as I know)
I don't see wsi surviving in the light plane market much longer though.   As soon as Chelton starts supporting xm
and xm starts covering Canada I think it will be over for wsi.

If you like xm, consider mounting a 396 into your panel.
If you like wsi, I would go with an av100 and an es800 mounted on the panel

Colyn