I own the Zeiss Victory Diavari 2.5-10X42 which is mounted on my custom Mauser '98 in 7x57.
I chose the 42mm objective as it allows the 30mm tubed scope to be mounted slightly lower to the rifles axis, thereby preventing the whole package from being top heavy and so unbalanced.
If I need maximum light transmissin I turn the magnification down to 6x. With the FFP there is no change in POI. At 4x my son has shot bushpigs in RSA at last light when they were invisible to the naked eye.
You will not be dissapointed with either scope which have magnificent optics and construction. I doubt in the field you will be able to tell the practical difference between the 42 and 50mm objective.