Up to you - as I said previously, both are run over the same route, both cost the same, the T-shirt you get at the end is the same in both cases and entrants in both have the same chance of winning a spot prize.
The main difference is that the Challenge sets off over an hour earlier and is, essentially, non-competitive. If the goal is to get round and enjoy the scenery without having to worry about how long its taking you then the Challenge is the better option. If, however, you're confident that you can cover the race distance within the time allocation (bearing in mind the hills and the off-road running surface - both of which slow you down) then enjoy the lie-in and do the race.
Based on that 10k time, I'd suggest that you could compete in the race. Maybe the best plan now (with the event still over two months away) would be to enter the race, see how the training goes and, if you start to get cold feet, swap to the Challenge towards the end of March - you'll be allowed to do that without it costing anything; from memory, swapping from Challenge to Race probably won't be allowed.
Whatever you decide, you'd best get in quick - when I looked at the website yesterday, it said this was 90% full.
Look forward to seeing you there.