I've toyed around with buying a Nikon P900 for various things including planes.
The 2,000mm equivalent focal length can come in handy in good lighting conditions.
For a "compact" camera this (or one similar) would be what I would get.
But this depends upon the type of plane spotting you do.
Do you only go to airports and shot them landing and taking off?
such as ==>
Air02-09
or small aircraft such as this =>
Air_20150621-04
Or do you like to determine landing patterns & flight patterns and catch them without their wheels down, and maybe making a turn ?
==>
Air20150517-20
I've shot planes 9+ miles away and cropped the image to bring it in. The image was better on a Nikon d600 than a Nikon d7000 crop camera both using a Tamron 150-600 @ f/8 and Sigma 150-500 when I was doing testing.
This is an example of a 9 miles (14.5 kilometers) away, Korean Air A380 - it's grainy unfortunately due to the massive cropping ==>
Air20150517-66 Airbus A380 @39,000 feet - 9 miles away
Or this about 7 miles away (11.2 kilometers) ==>
MD-11 @10:45am 4/12/2015 @ 33,000 feet
But it's always good to figure out flight patterns (dependent upon weather) and what planes fly from where and to where. I use this online application - example to Detroit, MI
Detroit Metro Wayne Co Airport (Detroit, MI) KDTW / DTW Flight Tracker ✈ FlightAware
I can see what's coming, or for instance select only FDX (Fedex) planes are they normally use DC-10/MD-11s which are neat to photograph.
But it all comes down to what type of plane spotting you plan on doing. If it's just at the airport then your options are quite numerous. The further out the plane is from your location then you need a larger zoom lens type camera.
Then you need to determine if you like to post process your images. For instance the P900 does not use RAW files, only JPEGs, thus that may be an issue for you.
My biggest issue lately is trying to get the international space station. Which is full of problems.