There are not a lot of new options for flash that is sub-$150, though the Metz Mecablitz 20 C2 is one of them and Metz makes a good flash, though I haven't used this one personally.

In terms of flash shopping, considerations would include:

1. Recycle time (how fast is the flash at being usable again once fired).

2. Guide number (at full power, what's the range of the flash at ISO 100, the higher the better).

3. Power supply (e.g. what type of batteries, most are AA).

4. Ability to control the flash power. Sometimes you want to be able to dial down the power and decrease the duration of the light burst.

5. TTL support for the camera.

6. Wireless support and/or sync cable support.

7. Trigger voltage. Some older flashes can have higher voltage and that could cause damage to your camera.

However, some of these considerations are less an issue for basic flash use for taking candid shots of friends and families at parties, etc. Also, given your budget, in some ways you're going to really just look at getting the best you can buy. I'm a fan of hitting Craigslist for stuff, but you may want to look at going new. You can, of course, get a more powerful used flash, but there are risks (see item 7 above) and it's really much better to go in and get some expert advice at the counter if you're not sure.

I can't say on the Aurora soft box, I'm afraid.

Edit By the way, if you do go used, take a look at this page on voltage by flash as it applies to the EOS line.