I had a similar dilemma to you.
I struggled with using A5 Filofax format because it was the best size /capacity for work, but wanted something smaller to carry around. There is more relevant stationery available at Personal size, but then there isn't enough room to manage everything for work. I solved the problem by splitting into 3 Personal size organisers. This gives me more room for the calendar and project planning sheets.
Here is my system.
1 x Compact Guildford
1 x Personal Guildford
1 x Personal Zipped Guildford
1 set of Filofax Month dividers
1 set of Daytimer tickler dividers
1 set of Filofax A - Z Dividers
Next Action Sheets from DIYPlanner
Waiting on Sheets from DIYPlanner
Project List Sheets from DIYPlanner
Project Detail Sheets from DIYPlanner
1 x transparent top opening envelop
Custom project plan sheets based on the Filofax Professional TM stationery but for personal format size.
I use the Compact size Guildford for this, which is filled with note paper and the transparent envelop. The compact is a good size for use as the inbox, more capacity than a slimline, but still slim enough to be carried in a pocket. This means I can carrry it wherever I go. You could use a pocket size organiser instead.
The Personal size Guildford has sturdy 23mm rings, and so is tough enough for everyday use as my GTD organiser.
In the Personal Guildford:
The blank dividers are labelled as follows:
Areas of Focus
Tickler (Someday / Scheduled)
The Next Actions section is filled with Next Action Sheets from DIY Planner, the Projects section with Project List Sheets, the plans section with Project Detail sheets, and the Waiting on Section with Waiting on Sheets, all from DIYPlanner.
In the Tickler section I have the Month dividers and the Daytimer Tickler dividers to set up a tickler file.
In the Zipped Guildford, I have the A - Z dividers and the diary for recording what has happend, so that I can refer back to it.The Guildford Zipped has a larger ring size (30mm), so perfect for filing. The zip also means that I can drop items in loose without punching holes in them.
The Inbox is carried everywhere. Notes from meetings, sudden thoughts, anything that occurs to me to do, actions in meetings are all written in it. I use the transparent envelop to store small items, receipts that need to be checked or recorded, tickets to file, business cards etc.
But nothing stays in the Inbox for long because when I am back at my desk or in a position to work through the Inbox and either file notes into the Reference folder, or process them as actions through the GTD Organiser. I work on everything out of the GTD Organiser. All NAs, projects etc are there. And I always remove the pages from the Inbox when the notes / actions are transfered. It's really satisfying.
If I need to take certain items with me when I am out I just transfer them back to the Inbox temporarily to work on them (OK, then it isn't an inbox at that point...) but this is just temporary.
I find I have more capacity now than the A5 organiser, access to better stationery, more portability, and it is all much more flexible.