I own the Sweet Sixteen and love it. I've seen the Viking Platinum at quilt shows and suspect that it is made by HandiQuilter - too many similarities in the table design and Trustitch is the HQ stitch regulator. The main difference in the machines appears to be the placement of the thread spools/spool holders.
Ultimately your decision may come down to access to dealers/support. HQ has excellent customer support by phone, in addition to the network of dealers. I see by your location that you live outside the US. I don't own a Viking so I can't speak to what their remote customer support is like.
Cheers, K