The Canon has a swing out viewfinder.
The Nikon doesn't.
The Canon your lenses will be the EOS EF and EF-S lenses.
The Nikon is most lenses made from the 1950's forward.
The cameras are smaller which may be what appeals to you or the price range.
If you can hold out a bit longer and go Full Frame I would. APS is APS, Full Frame just larger, more image to work with. If going on the Pro End, but the Canon will be stuck with a the EF series lenses.
Again a FF Nikon will use ALL their lens line.
. . .
NOT correct.
It is a common misconception that the Nikon F mount lenses are fully backwards compatible. Not so.
The MOUNT itself, is the same mount from the 1950s.
The complication is in the communication between the lens and the camera has changed/evolved several times, and this is where you have compatibility issues.
AutoFocus. Nikon used THREE different autofocus methods.
AF and AF-D used a mechanical drive. The AF motor is in the camera. The D3xxx and D5xxx do NOT have an AF motor in the camera, so CANNOT autofocus these lenses.
AF-S uses an electronic AF. This is what the D3xxx camera uses.
AF-P uses an electronic AF, like a next generation of AF-S. BUT it has to be used with a compatible camera. On a not fully compatible camera, you cannot even manually focus the lens, because it is "focus by wire."
The AF-P lenses is compatible with the D3300,
only with the latest firmware.
Metering.
Pre-AI, these used the "ears" on the aperture ring, to link the aperture ring to the camera.
AI, these used a tab on the back of the aperture ring to link to the camera. So you don't have to do the "min/max" twist that you have to do with the pre-AI lenses.
The D3xxx does not have the mechanical connection for the pre-AI or AI lens.
G, these electronically linked to the camera. The D3xxx only fully meters with the G lens.
Aperture
Pre-AI and AI. These have a physical aperture ring on the lens, and you control the aperture on the lens, NOT from the body.
AF lenses. These have a physical aperture ring on the lens, but you CAN control the aperture from the body. You lock the aperture ring on min aperture. But remember, these lenses will NOT autofocus on a D3xxx camera.
G lenses. These do NOT have an aperture ring on the lens. You can only control the aperture from the body.
There are other complications where some lenses CANNOT even be mounted, as you can damage the lens or the camera.
As a Nikon guy, I hate to admit it, but he lens/camera mount is where Canon beats Nikon.
If I buy a Nikon lens for my Nikon, I have to consult a chart to see if that lens is compatible with my camera.
As far as I know, ANY Canon EF (FF) lens can be used on any Canon EF camera. And ANY EF (FF) or EF-S (APS-C) lens can be used on any EF-S camera.
The exception is, the older manual focus breech-lock FD mounts are NOT usable on the later EF/EF-S cameras.