Difficult decision, 55/50001 more commonly known as a Bardic Lamp, is the older of the two, its a highly reliable lamp as AlexS says. It comes in two versions, the 3 aspect (R, G, and W) and the 4 aspect (R, G, Y, and White).
http://infra.unipartrail.com/assets/pdf/267.pdf
The Halo Plus 2 HP-11R2 is the modern eqivalent if the bardic lamp and is becomming the satandard issue lamp around the railways, I personaly prefer the HP-11R3RT which is the 4 Aspect version. The halo is again a very good lamp.
http://www.furneauxriddall.com/haloplus.html
The decision between the 2 lamps depends on what you want it for, how much you'll use it and how much you're willing to pay. I picked up a 4 aspect Bardic for £20 and have found it useful for my work on the ELR, Several of my friends have Halos, Both the HP-11R2 and the HP-11R3RT, and find them just as good as Bardics and a lot lighter to carry (but usless for self defence and as a hammer!!), so the choice is yours. (or get both!)
The other option is -
This
A snappy little torch, 3 colour with a belt holder, very useful if you need a lamp but for some reason havent got a bardic or halo with you!
http://www.railchat.co.uk/phpBB2/viewtopic.php?t=10355 - this is a good reference, the good and bad points of both.