score:4
Lonely Planet actually has three separate guides for Rome:
My rule of thumb used be that I'd get the pocket guide to carry around for short trip, or the city guide if I'm going to stay for a long time and/or want to do exhaustive research beforehand. These days, though, I rarely bring along guidebooks and only use them for research and getting an idea of the place before I go.
Obligatory disclaimer: I used to work for Lonely Planet, but I no longer do and this represents my opinion only.
Upvote:1
In the past I would have said the big one, as it contains a lot of practical information and mostly that on hostels was hard to get any other way.
But as you do use booking.com hostel information is easy to get.
I would likely go for the smaller version and look up information on internet, or do without a book and make notes from internet.
The big LP books get very heavy in the longer run, it often was the most heavy single piece in my luggage.