Neither! I'd take a deep level tube, as mentioned in the title! The interchange at Oxford Circus is cross-platform.I can either take the Circle line or the 36 or 436 bus. I know the tube will be crowded but how about those buses?
That would take ages (and involve a further change as Southern do not call at Willesden Jn)! If the objective was to avoid Victoria, then you would change at West Brompton.By any chance travelling from Croydon? Could always take the train up to Willesden Junction then Bakerloo back down...
I'm thinking 0830 from East Croydon - Victoria (0848), then the 36/436 arriving at Paddington around 0921 in time for the 0930 train to Slough.
08.23 to Farringdon (arr 09.00) then any amount of Met. or Circle line trains to Padders. You'll easily get the 09.30 from Padders. Having done the journey from Paddington to Croydon many times via several different routes I've found that one to be the easiest.