I have run into some similar situations with other adapters such as POP3, FTP and SAP(dot net connector version) since they are not supported in a 64 bit host instance. I have not noticed any negative effects of running Receive or Send operations in a 32 bit host instance and running orchestrations in a 64 bit host instance.
In terms of 64 bit performance here are a few advantages: “…faster messaging/orchestration processing or the larger addressable virtual memory of 64-bit mode for large BizTalk message mapping or other memory intensive operations.” from http://social.msdn.microsoft.com/Forums/en-US/biztalkgeneral/thread/fe3c54df-548f-4279-b01b-816e5c2e52c3