Go w/ Larger HD
Seeing as how you want to try out a lot of things, you may want to just go ahead and do the larger Harddrive. 30 GB would work as a dual boot, but not if you're filling up one of the partitions.
You [i]could[/] use the external idea, but I don't know how you'd get it to boot off of that. Perhaps you could use it as extra storage in a dual boot scenario. Just make sure it's not formated as NTFS, Linux doesn't do well with NTFS in the writing sense.