In the pivotal PhIII trial of ipilimumab, Grade 3 (severe), 4 (life-threatening), or 5 (fatal) AEs occurred in 6.7% of patients treated. In other trials, it ranged from 10-26% [1-3]. In the real-world these may be even higher [4] and then there is the cost.
So based on that I think it makes sense to encode anti-CTLA-4 over anti-PD-1 in the virus. Other reasons are that it blocks Treg activation and/or inhibition of T-cell activation [5,6].