whereabouts in oxford will you be living - this will make the crucial difference. For example, if you live on the donnington bridge side of the town centre, your best bet might be the a34/M4/a4. But if you live in rose hill, the back way through wallingford might be better
if you live on the Posher (!) side of town then coming through the town centre to a34 will be just stupid, so taking the road through to caversham might be better (depending on where you are in tilehurt, cause youd have caversham bridge to consider!)
Both of these journeys can be long, especially at rush hour times. people frequently do stupid things on the a34 and 1 lane will be closed etc. I work in tilehurst, and had to come to a course at oxford brookes and found the "best" (not necessarily the shortest) way for me was the a329 then cut off so you're going through pangbourne. I liked this route because it was more interesting than just drive drive drive on dual carriageways. it used to take me about an hour but i left earier and finished later so was never going through at 8/5 pm
What job is it?