Bava Batra 30: Again, the Burden of Proof

When Rava contradicts himself, and Rav Nachman does too: upon which party to a dispute has the burden to prove his claim? The claimant vs. the temporary person who maintains the current presumption of ownership. Also, the case of one who established a presumption of ownership over time. Plus, when there are witnesses that might help one avoid the courts - which might seem to be easier to solve..