Strictly speaking, there is a slight diffference in nuance between the two. If you asked, however, if most native speakers actually used the two forms "correctly" in different situations, the answer would definitely be negative.
The difference is in the degree of humility expressed. 「僕ら」 expresses a somewhat higher degree of humility about the speaker himself and his gang than 「僕たち」 does.
Would we expect Japanese-learners to know this and use the two forms according to the occasion? I am not even going to answer that because y'all should know we are the kind of people who will praise you frantically if you can say more than two Japanese words with the thickest foreign accent.
Seriously, though, I feel that native speakers actually know deep inside about that degree of humility thing even if we might not be careful enough to use the two forms differently in our daily lives. I say this because I do know that very few of us, if any, would ever use phrases like 「[先生]{せんせい}ら」、「お[師匠]{ししょう}さんら」, etc. where we know that we are expected to show respect. 「僕ら」 is different because it refers to the speaker himself and his company.