Is there a major difference between cow and pig heart valves for replacement?

Not really: Both offer the advantage of not requiring "blood thinning" on a long term basis. Unlike a "mechanical" valve, cow and pig don't last longer than 7-12 years(with a few exceptions). Otherwise, the outcomes in both valves are generally the same and for all intents and purposes, either one is a reasonable choice in a patient who does not desire blood thinning.

Probably not: Porcine tissue valves are made from actual pig heart valves while bovine valves are constructed from cow pericardium. The decision of which to use is often by surgeon's preference. In a recent european study, longer followup of bovine valve appeared to have some benefits over the pig valve (dalmau et al: http://www.Ncbi.Nlm.Nih.Gov/pubmed/21193320). You should discuss this with your surgeon.