Ride them both
Do you want to add mudguards, pannier rack?
How and where will you be using the bike? If you are actually riding cyclocross and thus likely to crash, alloy might be a safer choice. If just for roads and paths, it's horses for courses
Carbon isn't better, it's just different. Depends on preference. Personally I'd take steel over either if only bike
Is everything else identical spec wise
Check the terms of your insurance policy, depending on limits that £300 could mean a hike in premiums