So my quick answer is that both 141/Ionisation and 146/Optical are good and much much better that not having one.
It does not matter to your system if you have a mixture of 141/Ionisation and 146/Optical.
Whichever you get will impove your safety, so if on budget get 141/Ionsiation. But best to get 146/Optical one if you can afford it.
Also, you do not need to change the one you have upstairs - if you are happy you can update that one to correct version when/if it starts to fail.
Longer answer is
ei141 is Ionisation costing about £15
ei146 is Optical costing about £25
ei141 --- £15 --- Ionisation smoke alarms (stairwells, landings, offices):Ionisation smoke detectors have traditionally been used throughout properties for many years. Designed to react quickly to fast flaming fires, ionisation smoke alarms are most sensitive to small particles. When fires produce little or no smoke but the fuel is subject to rapid combustion, the ionisation smoke detector is the quickest to sense its presence. These fires tend to originate from materials such as paper and clothing. It's quick reaction to small particles can make the ionisation smoke alarm prone to false alarms, particularly when located near kitchens. Because of this, it is recommended for ionisation alarms to be installed in stairwells, upstairs landings and offices. This should avoid the occurrence of nuisance alarms when cooking or burning toast!
ei146 --- £25 ---Optical smoke alarms (also called photo-electric smoke alarm) (bedrooms, lounges, offices, hallways and landings):
Less prone to false alarms than ionisation,
optical smoke detectors are slightly quicker at detecting slow smouldering fires that tend to produce a lot of smoke. These fires tend to originate from upholstery type materials or over heated wiring. Also known as photo-electric alarms, this quick response time to smouldering fires is down to the optical smoke alarm's high sensitivity to large particles in the air. The optical sensing chamber effectively "sees" when smoke is present, as the large particles block and cause an infrared light to scatter. Optical smoke detectors are suitable for installing in bedrooms and living rooms where soft furnishings and cables are common place. They can also be installed in downstairs hallways as they are significantly less likely than ionisation alarms to sound falsely when cooking fumes are present from the kitchen. Optical alarms are, however, not suitable for areas open to the elements and very dusty environments.
https://www.safelincs.co.uk/smoke-alarm-buying-guide/
