When going out, I use the cartons. They are sterile, so can be stored at room temp, opened when you need them and poured into a sterile bottle to feed. You can warm the bottle in hot water or just feed at room temp. Personally, I try to plan going out around feeds, so I don't have to do it often. My daughter can go 4-5 hours now when we're out as she's too distracted to be hungry. But when I do need to do a feed when out, the cartons are so much easier than making up a bottle (I did that for a few weeks, too much of a hassle). They are more expensive, but if you're only using them a couple times a week it's not a big deal.
For nights, I just make them fresh. It doesn't take too long for me to boil the kettle, make the bottle and cool them down. It only takes a few minutes and my husband does the nappy change and sits with her til it's ready. My daughter rarely has a feed at night anymore so I hate making them in advance because it goes to waste (I don't like to feed anything that's been in the fridge 12 hours, which it would be by the next morning). But if you know you have night feeds to do, I'd just make a bottle or two up in advance, cool them and store them in the fridge, then warm in a bowl of boiled water. That's certainly safer than using water that isn't warm enough to sterilise the formula properly. And it saves money on using the cartons all the time. But however you make them (in advance or fresh), they should be cooled quickly in a bowl of cold water and either fed right away or put in the fridge. It's best not to just leave them out at room temp to cool.
