You don't need to alternate. FCL710(b) If the variant has not been flown within a period of 2 years following the differences raining, further differences training or a proficiency check in that variant shall be required to maintain the privileges, except for types or variants within the single-engine piston and TMG class ratings.
Not that it makes much difference. I intended to renew on whichever one I hadn't flown much but the truth is I will renew on whichever one is cheapest..