Acton Depot - tour or open day?

It really depends on what you want to see. The private tours visit parts of the depot which are not always viewable on open weekends, in particular the photographic store, which is climate controlled and hence can't have people going in and out all day. The poster, artwork and small objects stores may or may not be open (by recurring tour) on open weekends, depending on availability of staff, but will definitely be visited on a tour.

You'll get far more time to explore the main shed on an open weekend than you would on a tour as there will be enough staff to allow you to wander around, rather than having to be within sight of two guides at all times. You're also more likely to get more in depth information about a given area on an open day as usually staff are rostered such that each ends up in an area they know about (and even if they don't there will be someone in the building who does know), rather than just having the guides to ask about everything.

Depends on whether you're also interested in the 'extras' that you normally get at an Open Weekend (trade stands, visiting vintage buses, etc). The next one is London's Transport in Miniature on 13/14 March, so there'll be model railway layouts there as well.

Of course, one other good reason for coming to an Open Weekend is that you can come have a ride on the railway ;-).