Choosing a reputable provider instead of virtualization method will be a better bet.
If they're both offered from the same company, I would rather go with Xen due to how OpenVZ currently manages memory -- that is until the new OpenVZ kernels are introduced and brought mainstream. Takes away all the mess of memory used vs allocated, burst, etc.
It depends on what hardware the node is using and how well the server is managed. If both of them are managed well and running on good hardware, and OpenVZ isn't oversold, OpenVZ will outperform Xen every time.
I vote XEN just because it would save the fact that it be very noticeable if they where reselling, and the resources is dedicated to you (Dedicated Ram, SWAP), etc and most cases you have a much better service using XEN then OpenVZ.
XEN is a great platform, and provides a very solid and reliable service. But it also depends on the provider/hosting company your going with.
Do you mind providing them?
But I still vote XEN. (And I'm a OpenVZ fan, but with my work been rolling out and doing well with xen and kvm i've came to enjoy and love xen. over openvz)
If you have extra ~$10 to spare, try both VPS out. Oversubscribed Xen server will perform poorer than moderate-subscribed OpenVZ server (or vice versa). It's really hard to tell from what you described.
No expert on django but the app I deployed using nginx + fastcgi takes around 20mb per process. I can pretty much fit LNMP + Django under 100MB without any issues at all. Then again, your app might be far more resource intensive. Until OpenVZ gets a proper memory accounting, I'd stick to KVM/Xen.
I also would be prefer Xen or XenHVM over OpenVZ if price and specifications are the same. But the reputation of the VPS webhosts is also very important and should be taken into consideration.
As you don't mention the webhost or webhosts behind your selected offers it's difficult to determine which offer is more recommended. But you should definitely choose 512 MB RAM whether it's Xen or OpenVZ.
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