Thank you for your enquiry into the differences between our Galaxy and Mercury car seats.
As you can see below we have put together an in depth response for you regarding the primary differences between the 2 car seats which accompany our travel systems.  
I hope this will answer any questions you may have had.
1 - Increased Safety
The Mercury car seat is tested to the latest, much more rigorous I-Size safety standards which includes additional side impact testing, whereas the old standard for the Galaxy car seat included just front and rear impact testing only. 
Also, the new I-Size European safety standard ( R129)requires babies to be seated rear-facing for longer as this has been proven to be safer in the event of a collision.

2 - Longevity
The Galaxy car seat is suitable for babies up to 13kg which is approximately 9-12 months.  The Mercury I-Size car seat is suitable up to 80cm in height which is approximately 15 months and ensures babies sit rear facing for longer. 
This has resulted in the Mercury being a bigger car seat with more space for baby whilst harnessed inside.  (Ages + weights are only a guideline).  

3 – Premium Quality
The Mercury I- Size car seat is a high end specification product with better fabrics, ventilation system and reinforced shell/plastic technology. The Galaxy however is a mid-range car seat in comparison, although still tested and approved to ECER 44/04 standards.

Below are the links to each of these car seats for you to take a further look.

Mercury i-Size Car Seat | Car Seats | Ickle Bubba