Running Ubuntu under VirtualBox vs natively
Running an OS natively vs running it through a VM both have their strong and weak points.

Running it natively will give you better performance because you aren't running two OS's at the same time. You're native install will have all the RAM and CPU cycles to itself.  If you have extra HD space that you don't mind partitioning to use for the install, its great, just be sure to give yourself enough space in the beginning because expanding partitions can be a pain. There are many guides to installing Ubuntu on your mac all over the web, usually involving rEFIt. This can be a little tricky depending on which mac you have, but if you follow the guides out there carefully, everything should go fine.

Running it in a virtual box can be great though because it is the easiest and most hassle free way to get ubuntu on you mac.  This is what I would recommend.  Having it in a virtual box means that you don't have to partition your drive, it can usually be moved easily between Macs, and it doesn't require you to reboot your machine, you can even delete it if you hate it and no repartitioning to get your space back.  And best of all you can still access the mac side at anytime and switch between both seamlessly.  The downsides of this are usually performance when it comes to heavy things, if you are doing regular things (web surfing, programming, document editing, etc) though then it won't be a problem, if you plan on doing super CPU, RAM, GPU intensive things on your ubuntu side then you might want to consider the native install path.  It is really up to you though. The more RAM the better. Most machines come with 4GB which is good for running OSX Lion and Ubuntu side by side.

Personally, I would recommend trying the virtual box install first, seeing how you like it and how it performs. If everything is great, no need to try the native install.  If not, then those guides all over the internet will lead the way.

I hope that this answer helps you out.