There is no simple answer to this question, both types will do what you want.
Often the processor type is in a sense hidden from view, since you are programming not in machine code, but typically in a higher level language. I chose to use AVR years ago, and have no regrets, but that was only because they had Electrically Erasable parts early on. Not a reason for choosing them now!
Microchip has acquired Atmel, so who knows what will happen in the future, depends on how much control Microchip management will impose on the technical and financial aspects of what was an independent company.
Make a choice based on the I/O(dependent on what sensors you use and if analog or digital), speed, power consumption, cost, and any other parameters that matter etc, make a sound technical case, and I doubt you will regret your choice afterwards. See if the chosen processor (family)has a ready made development board, this will make testing simpler.
Language, C is most commonly used, but there are plenty of other languages available, partly depends on how much effort you want to put into learning a language, some are easier to learn than others. C is closest to the hardware is a common argument for C, but it can be a language of obfuscation as well.
Again, some people prefer an IDE, others a command line, a good IDE can be useful, but it is usually more abstract than using a command line. Simulators are often part of an IDE. It may in part depend on if you are a Windows user, or have come from a Linux background.
Free compilers are available for both processors, some will run with or without an IDE.
Go to both Microchip and the Atmel sites and have a look at what they have to offer that lines up with your requirements, and go from there.
Additionally, both have development boards that carry a range of their processors, these are also available from many other companies as well as the chip makers.
There are forums which offer help, and where you can see the sorts of problems that users, particularly new ones are having, and get some assistance.
Best I can offer, others may be more opinionated, and possibly correct,
Cheers,
Richard
