Sensor performance difference is negligible beyond ISO400. It's barely significant, at one-half stop or less, between ISO100-400 when comparing the D3300 and D7100. At ISO 100 only, the D7200 is about a full stop better than the D3300 and a half stop better than the D7100. From ISO 200 and up, there's very little difference between the D7100 and D7200. They're all old APS-C cameras from 2015 (D7200), 2014 (D3300) and 2013 (D7100), respectively. The image quality doesn't seem to be that much different between any of them

The main differences between the D3300 and D7100/D7200 are in the autofocus (AF) systems and the overall camera build quality of the D7x000 series bodies.

• The D3300 has a limited AF system with only 11 AF points. The D7100/D7200 have 51 AF points including several centrally located "cross-type" AF points and a center AF point that works with lenses as narrow as f/8.

• The D7100/D7200 will feel like a much nicer camera because they're bigger, heavier, and have larger optical viewfinders with 100% coverage compared to the smaller D3300 viewfinder with 95% coverage.

• The D7100/7200 has better environmental sealing that makes it more resistant to dust and weather. That doesn't mean they are "weather proof", though.

• If you find that the D3300 AF misses a lot for the type of shooting you do, the D7100/D7200 may help increase your keeper rate if the issue isn't your technique (which it usually is).

• If you use longer focal lengths a lot, the D7100/D7200 with AF Fine Tune might make a real difference if your lens(es) and D3300 camera aren't well matched in terms of manufacturing tolerance variation.

• The screw drive AF motor built into the D7100/D7200 body will allow you to use AF with older Nikon AF lenses that do not have a focus motor in the lens like those found in AF-S, AF-I, and AF-P lenses. The D3300 does not have an in-body AF motor so AF lenses will need to be manually focused.

• The D7100/D7200 have dual memory card slots, the D3300 only has a single memory card slot. For critical shooting situations, saving images to two cards might protect you against a defective memory card.

• The D7100 and D7200 only have limited compatibility with AF-P lenses. The D3300 is fully compatible with AF-P lenses. AF-P lenses were first introduced in 2015.