Vic is OK, only 2 stops to Oxford Circus which is a pretty good central starting point for a day out.I'm afraid neither is rubbish.
Clapham serves as a useful point to get to Victoria, and everything bar Gatwick Express will stop there to help you get there. But Victoria is far from the most connected of London terminii for most of London and will require a change of line if using the tube (bus is better but obviously slower, and the whole place is a bloody building site). Slow trains to Waterloo stop there but most of the longer distance ones don't stop there, many are Woking then Waterloo, so if you were coming up from anywhere to the southwest of Woking (e.g. Portsmouth, Southampton, Waymouth, Salisbury) it's actually not that convenient to get to Clapham compared to just going into Waterloo.
As for the Underground, well there is no Underground station at Clapham (yet) and nor is there one conveniently close by (the Clapham ones being further east along the High Street. As the crow flies the nearest tube station to the Junction is Clapham south, a mile and a half away on foot. There is the Overground, however again for Central London it's terrible, involving most likely a change at West Brompton for the District line then probably another at Earl's Court to avoid being sent up toward Edgware Road.
Also think few people understand how walkable London is. If you're prepared to stroll about you can cover a lot of the City and get to see so much more. For example, probably a little over 4 miles will get you from say Harrods in the WE to Tower Bridge, if you want taking you through, say, St James Park, Green Park, Piccadilly, Covent Garden, St Paul's and the City.
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