It all depends on what you want out of the holiday @Naughty Cal. If you want to spend time sitting by the boat (in the April showers...) mopping up a bottle or two of wine and burning some fray bentos pies on a disposable barbecue go upstream. If you want to spend some time enjoying a bit of life in some wonderful Thames-side towns/villages go down.
If Oxford City is a must (and it is one of my favourite cities in all the land), you could always moor in Reading and hop on a 23 minute train... last train home leaves around 2300. That way you get to do loads of fun towns and Oxford, not just loads of country stops and Oxford. Bikes allowed on most trains... Depends what you want, but having spent the last thirty five years* up and down that ol' river in one way or another, I would say that down is probably more fun (c:
*scary now I come to think of it... studied Civ Eng at Kingston in the eighties, lived on Trowlock Island (row-boat/Bonwitco), moved to Henley for ten years (shetland 535, clinker ship's lifeboat conversion, home made marine ply boat called Duck, and, obviously, a floating 2cv), went up to Oxford Uni for a year (took the Shetland), got married and moved to Wargrave (NO boat, what does that tell me about marriage... didn't ski either...), separated, moved to Reading (house backed onto the K and A, Dejon 14 and a Viking 20), moved down to current place (many, many boats...arrived at current house in the viking). I am old Father Thames. Grandfather Thames now I suppose, and little Grandling is spending his formative months right here by the river ?)
Not in a rental? Just ram it into a tree... (one of my many many was a 44' Buccaneer Bounty that spent two decades taking parties of stags and hens families in and out of Benson...)