Hi Noelene,

I can tell you one thing for certain   - It is going to cost a lot more than the first purchase.

There are lots of makes out there and I don't want to get into which is best because I believe each of them has their own advantages and if these match your needs then that particular one will be best. Do as much research as you can beforehand is the solution. However no one can deny at the present time Nikon & Canon are the market leaders. 

It definitely is not all about megapixels there are plenty of photographers producing stunning A3 prints from 6Mp cameras. Both of the cameras you have mentioned are capable of producing professional quality A3 size prints. You will need to postwork to size but this is common practise.

It will make a huge difference what lenses you use. Do not economise in lens quality, have less lenses or choose a lower priced body but buy the best lenses you can.

Consider the other cost implications: New stronger tripod ? New software for RAW ?  New top quality filters for landscape work, New batteries, New memory cards, New camera bag, etc etc etc............

 

 

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html