On the surface less efficient if addresses are reused (which is often not recommended), but not dramatically so. Both end up with block headers plus UTXO.
Perhaps there are also issues around efficiency of how the history is stored with multiple Merkle trees (one per block) verses one Account Tree. Again not major, probably a constant factor.
Yes if there are straggler unspent transactions in old blocks then there will be some merkle branch hashes that need to be kept. Once all the transactions in an old block are spent that goes away and all you have is the header, which is exactly the same as MBC.
This may seem implausible if there are many transactions per block. Except that to keep the ledger from filling with dust and abandoned outputs, MBC suggests (though Cryptonite does not yet implement) some per-account demurrage (idle balances eventually decay to zero and can be pruned). If you do the same thing in bitcoin then you can clean up the old blocks as well. In the end they seem comparable.