Which UV?

Hello all!

I am looking for a bit of advice from you helpful people about what type of
UV steriliser I should buy. Right now I am considering several brands:

Rainbow Lifegard 25watt

Custom Sealife Double Helix
or
Emperor Aquatics 18 watt SMART UV steriliser

I want to lean towards the Rainbow because I can get the spare parts the
easiest, and I can connect it inline without a pump, but I am concerned that
the design of the housing will cause the water to pass mostly under the
bulb.
Thus, wht CS double helix is good for more exposure time, but I am not sure
of it's super-duper polymer sheath instead of the quartz, as well as having
to have a pump connected to it.
The emperor looks solid, but more expensive, and I would have to wait longer
to get it shipped.

Any suggestions or observations about my concerns of each unit??
Thanks!

PS- Going to use it on a 50 Gallon, but want a more powerful one so I can
use it when I get a bigger freshwater tank.

-Seweryn