Starting a business demands both passion and discpline. However, which of these two qualities is the "deciding factor"? Which is the quality that gets your business into profit, that keeps it going year in year out? You have to decide whether it is your dream and passion, or if it is just another job. If it's a dream or passion, then your motivation can direct your daily activities to a big extent. If you lack the passion, but see the potential profit, you will require a lot more discipline to move the business forward. I would advise anyone to combine passion with seeing profit opportunity. Just because you're passionate, it doesn't mean a profit opportunity can be realised. You still need discipline to do the "day in day out" running of your business to ensure it's a success.
For me, it's a matter of finding a passion you can turn into a business, while also realising that not every aspect of running a business is going to be enjoyable or exciting - that's where discipline is required.
Having said that, just because you think you can turn one of your passions into a business doesn't mean the market will agree with you. You need to research the market to ensure that this new business you're starting is going to provide a product or service that actually has a demand. Not only that, but you need to research price bands - what is the market willing to pay? Will those price points result in a profit for you, after you've paid all your expenses? And not just a meagre profit, but one that you feel is worth all the toil and trouble for?
Next, you need to manage your expectations! Your first year is really setting things up. You have ZERO customers from day one. You're not "hitting the ground running", but starting from zero. This means it will take time to not only make a profit, but to make any kind of serious revenue. In other words, you will need to be prepared to drawdown on savings - and/or take a busines loan out - to cover expenses in the first year. You REALLY have to mentally prepare for that, because one of the reasons why most entrepreneurs fail is due to them not managing expectations - they set their expectations too high and become despondent when they realise things are moving "slower" than they'd hoped for. They may not actually be moving slowly at all, but it's just that it takes TIME to get a business moving.
While passion helps you pick a business and identify a profit opportunity, it's discipline that ultimately seals the success of a business. Discipline is the real "secret sauce" that keeps a good business going in lean times, when you're feeling uninspired, when you're doing paperwork - this is the very thing that a lot of failing businesses will lack: the gumption to keep going. A business run purely on passion is vulnerable to the whims of its founder.
Saying one thing is more important than another isn't really the right question. Actually, passion and discipline are necessary ingredients to make a business successful.