+1 on KittyKats advice re making the change over - ideally your entire beneficial bacteria colony (living in the filter media) needs to be transferred from your old filter to your new one in order to avoid throwing your tank into a new cycle. Of course, it might not fit. In that case as recommended above, run both filters until the bacteria colony forms in your new filter. At no stage in the proceedings rinse your filter media with fresh tap water because the chlorine will kill your bacteria colony - instead always rinse excessive gunk out your filter in a container of water from your tank.
To answer the first part of your question, a fluval u1 sounds like a great choice for your 6 gallon tank (27 litres) since it's recommended for aquariums of up to 12 gallons. I've never had any probs with fluval stuff. Good luck with the switchover.