Laptop, without question. Tablets are ill-suited to creating documents, which you will need to do.
$350-400 15" laptops are a commodity these days. Check your Sunday newspaper flyers for ads from Staples, Office Depot, Best Buy, Frys, etc. You'll often find these weekly specials are cheaper at these B&M stores than online. If you're near a Micro Center check there as well. They always have specials in your price range. Indeed they have right now a Acer Aspire for $350:
Intel Core i3-2370M Processor
4GB DDR3-1066 RAM
500GB 5,400RPM Hard Drive
15.6" HD Widescreen CineCrystal LED-backlit Display
8x DVD-Super Multi Double-Layer Drive
5-in-1 Digital Media Card Reader
Intel HD Graphics 3000
10/100/1000 Network
802.11b/g/n Wi-Fi CERTIFIED
Microsoft Windows 7 Home Premium 64-bit)
A 15" laptop has a big enough screen and big enough keyboard to to work on, and today, they're not too big or heavy to carry around (yes, an Ultrabook would be even nicer, but outside your price range).
You ought to be able to find a i3 based laptop in your price range, and it will have enough real power to do whatever you need - certainly more than an E-450 would.