Welcome to Mini2 mate
IMO it is the better option due to the price. With the AMD conversion it gives you 195-200bhp for the same price, they quote £1650 to get 212bhp - which is still less bhp than the West Tuning conversion and almost double the price

I don't mean this in a bad way towards AMD - they are highly respected tuners but with this conversion you will get more BHP per £
Esentially the AMD and West Tuning packages change the same components but due to some clever ECU re-mapping a further 15bhp is acheived with the West conversion
I don't think this conversion will have any detrimental effect on your cars reliability. I have never heard of an engine failure caused by ECU re-mapping and pulley change - and this goes for any tuning company.
Cheers, LB.