Life Mobile is better.
Both Life and Virgin are 3G only but Life might have better 3G data speeds as they're directly owned by EE. Virgin might have lower priority on the network.
Plus with Life you can cap your contract. You cannot do this with Virgin. Go over your data allowance with Virgin and they'll charge you £2 a day to use more.
I suppose one of the perks of Virgin is that their tariff allowances include calls to ANY 08 number, including the most expensive 0844 and 0871 numbers.
With Life (and all other networks), only 0800 and 0808 calls are free, and 03 numbers simply come out of your allowances. 0870, 0871, 0845, 0844 etc are generally out of bundle calls and are expensive. Some charity helplines that have 084/087 based numbers may be free but this depends on the network.