Potomac Ave is closer as the crow flies, but they're both a decent walk. About a mile.

As an alternate option, you can hop on the EM-LP Circulator bus from either Navy Yard (you want EM direction) or Eastern Market (one more stop on orange, you want LP direction). It will drop you at 8th and M which is only a few blocks (0.25 mile) down M St to get to Maritime Plaza. From Potomac Ave, you have to walk nearly all the way to 8th St anyways and then backtrack to get across the traintracks/freeway.

Circulator comes every 10 minutes and is free when transferring from Metro.