DIFFERENCE BETWEEN ONE ARM and ROUTED DEPLOYMENT OF F5?

You also need to keep in mind that "one armed" configurations change your available bandwidth through the BigIP. If you have a single interface to your LAN, you will only get half of the throughput in theory. Make sure you scale links using LACP appropriately to ensure that you have enough bandwidth.

Client -> VIP = Ingress on BigIP Port SNAT -> Server = Egress on BigIP Port Server -> SNAT = Ingress on BigIP Port VIP -> Client = Egress on BigIP Port.