Tula's were made in serial number runs. The required amount were built and than CH marked. They were tested for accuracy and if they didn't pass, they were not drilled and scoped. That is why you see some Tula CH marked rifles that were never drilled. Their numbers are less than Izhevsk with 1942 dated rifles being produced in limited numbers before having to abandon the factory.
Izhevsk's were taken from serial number blocks of produced rifles and if they met sniper accuracy they had the machining smoothed and drilled and scoped. There are some Izh. rifles that have a small C mark located on the receiver but it is unknown if this is an indication of a sniper selected rifle that was not drilled. Most Izh. snipers have this C mark.
From a collectors standpoint, a 42 Tula is valued more than other models because of the limited production. The 45-47 Izh rifles are also valued more. Prior to 42, all snipers were made by Tula (probably with a few exceptions cause nothing is definite when it comes to Mosins) so Tula had a longer history producing the rifles.
Which is better? Doesn't matter, they all either shoot good or can be made to shoot good.