I was excluding the new OTS altogether. The lens sizes are 19, 30 and 50 respectively, that's one answer. The OTS is the smallest of the three. The XD50a is the better of the three as it pertains to vision. $ for each certainly another. I rented a 19a and think my OTS is better, but that's to be expected.
Go to Ultimate Night Vision and rent a 19a since you own the 50, if you don't like, you can use the rental amount towards buying a 50. If you want small or helmet mount, look at the
M&P-10+AAC Cyclone - Kel-Tec RFB+Griffin Sportsman Ultra Light 300
ATN OTS 336-70mm 60hz Thermal Monocular
ATN OTS 336-30mm 9hz Thermal Monocular
ATN NVM14 NV Monocular