I would determine which will get you the highest GH/s.

4 Antminers cost 0.324 BTC ($275), and give you 6.4 GH/s.
6.4 shares of cex.io cost 0.304 BTC and give you 6.4 GH/s.

If I had to pick one, I would pick cex.io then because it is cheaper and you can start mining now.

However, the best choice is neither. 6.4 GH/s costs 0.304 BTC, but it will mine no more than 0.13 BTC over several years, so you will lose about 0.17 BTC overall if you try to mine it.

Buy the BTC directly and you will come out ahead.