In North America the NES isn't a top loader and due to the mechanism they used the pins ware out, especially after 35-40 years. On top of that there is very little in the NES Library that's only on the NES these days. If I hadn't already invested in an NES and games, I'd just get the NES Classic and be done with it. If I wanted extra games I'd get collections like the Castlevaina Collection, The Mega Man one, and Disney Afternoon. You can extract the ROMs from these collections and put them on the NES Classic if you are willing to fiddle.

Meanwhile the master system is often times forgotten, but has a similar library for third party games. And what unique games does have are rarely found elsewhere as the master system version. Let alone the European releases like Sonic the Hedgehog.

With that said, I prefaced this with "Find games first" for a reason. The master system has a smaller library of games, and if OP was looking for games more like Dragon ~~Quest~~ Warrior or Final Fantasy, then a system with only Phantasy Star would be a bad choice.