It is worth noting that not all emergency luminaires offer the same light output, or indeed have the same build quality. Also, when comparing like for like, a self-contained luminaire may have a much lower light output than an equivalent slave luminaire.
This means you will need more luminaires to cover a given area. It is also worth noting that on-going testing and maintenance of a self-contained system will be more expensive and labour intensive than a Central Power Supply system.
The graph below shows typical light output (in Lumens) for various 8W bulkhead luminaires. (Yes, most new fittings are now LED but you get the idea. It is always worth checking the light output.)
As can be seen, the output increases from a budget 2 cell self-contained luminaire to a 3 cell version. And again for an AC/DC luminaire. The highest light output being achieved with the AC/AC luminaire fed from a static inverter.
Check the photometric data for details. If it doesn’t exist, it probably isn’t worth buying!