I am looking to purchase SD for Navigation and Planning
I also need to purchase an appropriate device(s)
Can you please share some of your good and bad experiences
Also ... do I need a contract like a mobile contract to use the device for GPS / SD.

For in-flight cockpit use a tablet (Android or iOS) is best. I don't see myself using any laptop in flight in GA.
I am very happy with my 7" WiFi only Android tablet with external Bluetooth GPS. If you have a desktop PC at home that would be ideal for planning and printing any paper charts that you wish to have on board. No need for mobile contract to use GPS function.

The question is Android or Apple

Both have their merits, but SD runs fine in both environments. Choosing an Ipad (mini) is fine, but you should consider to choose the LTE model, because of the absence of a genuine GPS receiver. You need an external GPS receiver for the WiFi only models, which is a big disadvantage. Brightness/Screen resolution/Endurance is fine except for occasional dropouts in hot environments - something we experienced on our last trip to Spain/Morroco. Choosing an android device you should focus on:

Performance - at least Quadcore 1.6Ghz+ SD is a demanding program pinch/zoom/map rotating needs sheer computing power
Resolution -  HD (1280 x 800) or better FHD (1920 x 1080), e.g. Samsung Tab S got (2560 x 1600)
Screen Brightness - you are working in an bright enviroment, so 100% brightness is more or less standard, this gives you about 4.5 hours of endurance on all devices, Apple ore Android. Consider to use an antireflective screen protector to avoid mirroring & fingerprints.
GPS Receiver  - should be capable of receiving Glonass, gives you ~ 20 satellites, so no more dropouts. I use my Tab Pro 8.4 even in an airliner without problems.
Screen size -  7 inch minimum, I use the aforementioned 8.4 inches. As a kneeboard on my thigh it works perfect.

My android recommendation would be: Samsung Tab S 8.4 Wifi/LTE - Sony Experia Z3 Wifi/LTE - maybe the Google Nexus range.

-mike