Muscowy, get 6 cores + 9 cores from mission, so its equal. Also it got almost all cores needed for russia unification and can easily get core on smolensk thru mission. It is highly unlikely thou, that novgorod will get core on musocwy, meaning that even if you conquer muscowy in 2 wars - meaning 10 years - and get all provinces needed for russian unification, it will take 50 years for those provinces to core - leading to quite big rebelions as those provinces are most, quite high on manpower, also, at best you will unify russia around 1460-70. And i think for novgorod, russia formation will be more likely to be in 1500, as lithuania tends to be pretty powerfull early on, and it is possible that it will be inherited - which means OP poland, that will got like 60k forces... or more. Even on its own, lithuania got like 30-40k, and if it manages to conquer some lands, it even sometimes goes to 50-60k. Also lithuania is both more wealthy than moscow and novgorod, but moscow missions increase wealth of muscovite provinces, meaning muscowy will got higher taxes, allowing to bring bigger forces, aspecialy as its sliders are better for army.
I started off by making friends immediately with Poland, Lithuania, and Sweden, which seemed to make the most sense since they were my biggest, baddest neighbors. Gave them all military access through Novgorod, spent a lot of my early income sending them gifts, and eventually formed an alliance with both Poland and Sweden. That solved the problem of them being tougher than me.
Muscovy, as mentioned, fell pretty easily. They'd been getting the snot beat out of them by both Poland/Lithuania and the Golden Horde for over a decade by the time I got a mission to vassalize them, so they were pushovers, even with Rurikovich generals running around. Vassalized them in 1415 or so, diploannexed them 10 years later, had core on all of 'em by 1475.
If someone as muscowy fare not as well as novgorod - well... i don't realt know how it would be possible. I mean i understand if weak player with muscowy don't fare as well as good player with novgorod, but same player should fare much better as muscowy than novgorod... unless he is very unlucky with the game on muscowy - get more nasty events etc.
It's just a different playstyle. I think a lot of players want to jump in and start immediately waging war on their neighbors. If you do that with Novgorod, you'll get pwned. With Novgorod, you really need to start off slowly and cautiously. Since I am by nature a slow, cautious, methodical player, it's worked out great for me.