I agree it depends on the space even if the loft is just 1 bedroom. It also depends on how many other bathrooms and ensuites.
Assuming first of all, it's a house with just one other bathroom;
If you can have a decent sized bedroom with storage and a separate bathroom which is a good size, then that is the best of all worlds.
This is because most of the time, no-one else will use it, assuming there is at least one other bathroom. That means it is private just for you most of the time. You can nip to it in the night, naked if you choose as you'll never see anyone else. Any issues with lights or smells are removed from the bedroom. On the occasion when other people are staying or in the house,not hey can access the loft bathroom if you want to make it available to them.
If however, having it separate will mean smaller bedroom or smaller bathroom due to corridor issues , I would go for Ensuite.
HOWEVER...if you already have more than 1 bathroom elsewhere in the house;
- probably go for Ensuite if you prefer it, as there are other bathrooms for random to use.
It's all about your personal situation and there's no right answer. If you have loads of overnight guests and just 1 other bathroom is a problem, having 2 which all can access is useful.
If you don't have loads of overnight guests and particularly if space is limited and separate bathroom will reduce space of bathroom or bedroom, go Ensuite.
It is always possible, on the odd occasion to let visitors use the ensure if you need to. For most people this would be rare and so not an issue. Also depends if you want or need a sanctuary from the rest of the family (kids) - people feel differently about this.
So...no right answer.