In my opinion your always better using something that is independent than shared which is why I would have opted for Xen over OpenVZ.
But as Spud has also pointed out. Depends if your host oversells.
Hosting is 100% a market you get what you pay for. Go cheap expect overselling, bad or non existent support, cheap hardware, cheap bandwidth.
If I was in the market for a VPS I would choose Xen however Ill never been in the market for a VPS. Dedicated is the only option for me.