I can speak to the M4A3 E2 kit. As a basic build it is a decent kit that builds up into a good representation of the Jumbo. As far as accuracy goes it tends to be a little bit off. The transmission cover is quite undersized and could use with some AM help. Also the turret is not quite shaped right. Both of these parts are available in resin from a company called Tank Workshop and are excellently moulded. Another inor flaw is that there should be a weld seam approx. half way back on the side armour plate. This can be easily added with a little scrathbuilding. I've already mentioned the sponsons so I won't touch on them here. A small PE set could be used on this kit and you'll have a few spares left over .... Eduard makes some very good PE sets and they are quite extensive. Remember with PE, only use the parts that improve the model in your eyes.
Forgot to mention the tracks ... Jumbos had extended track connectors so if you wish to go with AM tracks I would sgjest RHPS tracks that have them.
You could also replace the gun barrel with one from Jordi Rubio, either the 75 or the 76 would be correct as some were field fitted with the 76 (without the muzzle brake)
Enjoy the build John.
Happy Modelling and God Bless
Robert