The FX5 is ONLY a mechanical filter, its a big sponge filter basically, where as in the other (which looks like a white label aheim filter, or at least a copy) you get the UV which is good, and you get 500lph more.
another downside is that the FX5 is pretty big, but then there is a lot of filter sponge, so thats a benefit.
Personally I would go with the FX5 as its a brand you can trust, its worth spending the money on something that is proven and known, rather than something that is unknown could break and leave you with no where to go for spares. But fluval do all the spares you will need.
I looked at getting FX5s for my 120gallon tank, but I didnt have the cupbaord room for them and when with the 405s instead.
so anyway lets list the Pros and Cons
FX5
PROs
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Large filter space
Trusted brand
Know it does the job
Spares available
CONs (compared to the other filter)
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Only a sponge filter
No UV (but you can add one to the line)
Second hand could be used and abused
Only 1500 lph turnover
The Other Filter
PROs
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1 year gaurentee
UV Filter
Larger turn over than FX5
Sections for storage of different medias
CONs (compared to the other filter)
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Smaller canister
Unkown brand
Unsure of spares
Expensive for the filter
If you prefer the second filter over the FX5 Im sure you will be able to find NEW 405s for less than £75 (this would give you the pros and few of the cons of the other filter) I think I only paid about £60 for mine, thats was a few years back though. And they were bought online not from a shop.
Hope this helps.