lol, I love questions like this. The person asking is from Va., and one responder is from NJ, the other from the other side of the country.
Seems they are both correct about the wire size and ampacity, but have different ways of doing it. I have been a licensed journeyman inside wireman and journeyman residential wireman for near 20 years, and I agree with bigun, BUT huskinho might be right, because every jurisdiction (area) has different code requirements.
I might suggest that you ask a local licensed electrician what the requirements in your area are.
Personally I would not use USE aluminum feeder wire, but thats me, and your situation might warrant using aluminum wire. Is is a bit cheaper, but you have to increase the wire size 2 times to equal the ampacity of copper, and that can make it more difficult to work with, depending on the physical size of the panel, cabinet or disconnect you are using. Plus, by using pipe, it allows you to change wire at a later date if you ever were to change the panel to a larger one. If you do use aluminum, make sure to use a anti oxidant at every point of termination. Do not overtighten either, as this can cause the wire to sweat. Heating and cooling of the wire, along with this can cause the connections to loosen and begin arcing, again causing failure. Do not strip the wire by running a knife in a circular motion around the wire. This might score the wire and create a spot in the wire which might creat heat and cause premature failure. Also, make sure you bury either deep enough. An electrician can tell you what is legal in your area as well. Don't ask the guys at Lowes or Home Depot, they are just salepeople, not electricians.
And, around here, SER wire is not USE wire. SER=service entrance wire, and USE=underground service entrance wire. There is a big difference.