The downside is only if VA costs more.
There's heaps of upside : Sectors, extra status credits and potentially more points. ( Which reflects the VA earn and QR earn : Qatar Airways )
Also, I think Virgin's fees for changes / cancellations are much lower.
PS : I have heard from others that VA have been cheaper than QR. I recently bought flights and QR worked out cheaper, so no rule of thumb it seems.
