A 30 day SIM is just that - the minimum term is 30 days after which you can cancel it if you wish.
So depends on whether you're happy to commit to the Three one year deal and what the coverage is like as to which is better suited.
Three do offer out of allowance call blocking which Virgin Media don't at present
http://support.three.co.uk/SRVS/CGI-...se=EXT(EM38736)
Also you might need to get the handset unlocked if changing networks depending on the network switch.
I believe it should be possible to transfer the Virgin Media number to Three if that's the plan choice.