I have a Macbook mid 2009 and I wish to install Ubuntu in it, just want to make sure which version is good to go in it.
I would reccommend 12.04 (due to increased stability and support), unless you can cope with occasional breakage - then you might consider 13.04 which has been running well for me.
From a hardware perspective, either should work. Burn a DVD or use a USB stick to test it in live mode before committing to the install.
Firstly you have made the right choice in switching. I am running the latest 13.10 on a 2009 mac 5,5. Everything is working except the nvidia drivers don't play nice with my backlight control, so instead I am using the xbacklight app.
Installing on a mac can sometimes be a headache, so you may need to find workarounds for various features (like I have mentioned above). Make sure you back up your important data beforehand in case anything goes wrong.