As you can see below we have put together an in depth response for you regarding the primary differences between 2 of the car seats which accompany our travel systems.  
I hope this will answer any questions you may have had.
1 - Increased Safety
The Stratus 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 12 months. The Stratus I-Size car seat is suitable up to 87cm in height which is approximately 15 months and ensures babies sit rear facing for longer. 

3 – Premium Quality
The Stratus 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.

Stratus i-Size Car Seat & ISOFIX Base Set – Ickle Bubba

Galaxy Group 0+ Car Seat & ISOFIX Base – Ickle Bubba