I have setup a p2p solution between two offices using aes-128. When I had aes-256, I had a lot of retransmit requests.

That being the case, is there a preferred VPN solution between two RB2011 devices that will balance securing the data and provide good performance?

Thanks

Have you tried SSTP VPN? Between Mikrotik's you don't need certificates for SSTP site to site VPN

You question contain answer.
Any 128-bit encrypting will allow faster connection, any higher will decrease performance drammatically.
It is no big difference which concrete protocol to use, makes difference only encryption due to RB2011 do it only via CPU.
Some higher throughout will be with p2tp/openvpn/l2tp.
sstp usually little bit slower

As cheap solution to get higher rates with low price - is AES-CBC on RB750Gr3 (in real life you will be able to reach up to 100M) due to AES-CBC is hardware coded on hEX.