Live at the other end of Harpenden but Southdown seems really nice and everyone I know who lives there is very happy with it. The Grove schools are apparently v. good (infant and junior). Only thing I'd be wary of is not living at the extreme edge nearest St Albans - there's stuff in the local paper every year about children who live there not getting into Harpenden secondaries (although I think they do in the end, they have to go through all the stress of continuing interest lists).
Batford's 'less desirable' but seems perfectly nice. Anywhere other than Harpenden it'd be very desirable, I suspect... (the school down there, Sauncey Wood, used to be considered one to avoid, but I gather has been on the up for some time now. Again, anywhere other than Harpenden you'd have had people queuing up to get their kids in).
Southdown is nice - has its own little village centre and house slightly cheaper. I wouldn't make a decision based on schools - allocations this year have left 62 families without any school in the town including a number sitting on top of nearest primary.