I've got an air source heat pump in my office. During the winter, when the outside temperature is above 5 degrees it works really well, however as the outside temperature drops close to, or below freezing heat output drops off rapidly, which is a bit of a nuisance as that's when you need more heat indoors.
When used for heating, the outdoor unit blows out cold air, so even on days where the outside temperature is above freezing, because of the extra cooling, it can ice up, so sometimes upto 15 minutes of every hour it is in 'defrosting mode'. During this time no heat whatsoever is emitted.
Last year when the outside temperature was -10 to -15 degrees I had to leave it on 24/7 to maintain 21 degrees inside, even then, occasionally it had fallen to 19 first thing in the morning. If it was turned off overnight it was 16 in the morning only managing to reach 19 degrees after 6 hours.
I should add that the office is part of a new build (2007) on a business park. It has fabulous insulation and has gained an accolade from BREEAM. So it does retain heat that is put into it.
So I believe the advice you have been given is good. Go for the bigger unit especially if you don't intend to heat the conservatory permanently and need a quick warm up. Inverter control means that you won't use anymore power than you need in milder weather.