I use all my bikes, but it depending on what I'm doing on the day. Dr. Bike (carrying spare parts), one to one, trainee needs a bike, leading a snake, not leading snake, Level 1, 2 or 3, hilly route, long day etc etc
But if you are only going to have one bike, go for a lighter one. Because you will get on and off many times and have to move the bike around stop, start, park, lift, show this bit and that bit for your trainee. But don't use your bling Sundays best bike because it will be used/loved a lot.