Done both many times.
The SPAM Winter Challenge (The event formerly known as Salisbury Plain Challenge) has been ‘diluted’ over the past few years largely thanks to MoD being a little more restrictive over where they can route it. It’s now laps of a tame, tarmaccy course, but it used to be a proper epic event: sometimes frozen solid, sometimes a sticky mud-fest. And it’s all way-marked.
HONC is more 50/50 tarmac/tame off-road and is billed as more of an reliability ride. Weather’s not so much of an issue it being in April. In 2006 there was a lot of dry, baked hard field edges. Not way-marked – They give you a map and you have to find your own way round. Do not follow the guy in front…
Both events attract around 1000 riders. Of the two I preferred HONC (personally I think the scenery’s better – rolling Cotswold farmland – and there’s nice cakes at the halfway pointk, and there’s more climbing/descending :O)