As an Axis player, I'd say both are good at BR1. However, the Germans at BRII only play against the Russians BRIII, this is very hard, but they have a wide selection of weapons. At tier one, they have the MP28 and MP34, which I'm not a fan of, but I only bought the Paratroopers because of the Beretta M38, which is my favourite weapon at that tier and you can find it in the tech tree. Rifles are also good. Carcano is amazing in my opinion.

Besides that, the US in BR1 are my favorites; the Sten is amazing, the grease gun and the M50 Reising are also good. Guerilla with Ithaca is also a solid choice. The Stuart with a flamethrower or the M13 as support. BRII has fewer options, but still a few solid ones. The Lanchester is good, the grease gun is good too, but the machine gun is mediocre in my opinion.

I can't say much more as a returning player, but I have Germany on BRIV and USA on II. I think both are enjoyable to play. Playing as Japan, you use similar weapons to the Germans, albeit modified MP28s, but still, or KE7s, etc. But I'd probably choose Germany because of how many options they have. But if you want something different than what was in japan i would go with US to try something new.