Xenserver and great performance over iscsi are not to be said in one sentence...

Xenserver doesn't support nic bonding methods that increase throughput, in fact other bonding methods than failover are broken and can in some cases crash the host.
Your other option for providing more throughput would be multipathing with a multibus policy. Xenserver doesn't support this as well. If you do want to use multipathing (it works ok) you will have to configure your iscsi connections manually instead of using Xencenter.

Is you have to choose I would suggest using DDS...Openfiler is based on rpath linux and in my opinion it is a home storage toy... Rpath is a hell to maintain and Openfiler itself isn't really enterprise grade stuff, not in features and stability.