Couple of things to consider: 1. Price, 2. size and 3. OS - operating system & Apps, 5. Accessories.
1. Price. Apple iPads can be expensive compared to the Android tablets from Asus, Samsung LG etc.
2. Size range from 7in. e.g. Nexus 7, 8in. e.g. Samsung Note 8, 10.1, 12, Tad 4, LG G Pad. 3. iOS or Android. That is the question. If you consider apps into the picture iOS apps are still a little better than Android apps. 5. Accessories e.g. keyboard, is a little better for iPads than Android tablets - you will potentially see all lot more options for iOS iPads.
A lot to think about but better to go to the store and try them all out.
So if we consider your criterias:
Not too complicate: iPad
Ability to connect to keyboard: Both but iPad will have a little more option
Flexibility of taking quick notes: Both
Writing lengthy documents: Both - but iPad may have a slight edge
Also consider screen resolution and battery life.
Taking all these into consideration and the premiss that tablets are designed for content consumption e.g. web surfing, watching movies, and short response to emails etc.
If you do have a need for writing longer documents I would go for a thin notebook of the kind like an MacBook Air but you are than onto another price category.
If you are ok with Windows based products you can also look at the MS Slate products. Integration with MS Outlook is great but it's an MS product...