The decision to move is generally based on price (rates, fees, charges, etc), service (assistance with chargebacks, compliance, tech support), and integration/features/APIs. Another consideration is partner relationship.
Currently, we recommend APS first for leading in all 4 categories above.
CBS failed with one client were we recommended a switch from SPS but we weren't involved in the due diligence. 2 major issues caused immediate removal. 1) credit cards had to manually re-entered, and 2) workflow did not mirror SPS in that a CardID was required to process a transaction. Both are non-issues with APS where the SPS vault of CCs and Auths are transferred with a slick utility, and CardID is not required.