The Rena Filstar XP3 is a fine filter for its price point. I have Fluvals (FX5, 204), an XP3 and an Eheim 2028. If you are not after absolute silence, then Eheim is not the only option.
Canisters are actually very good at bio filtration. They're just not as good as wet/dry's, Fluidized bed filters are great at bio filtration, and they don't expose the water to air.
Anyway, the main differences between the XP3 and the Eheim 2028 I have found are:
1) Noise (Eheim wins)
2) construction (Eheim wins)
3) Initial flow (XP3 wins)
However, what I have found is that the Eheim filters the water more thoroughly, and also, is able to maintain a higher flowrate longer. That is, the XP3's rated flow reduces very quickly after it's put into service, whereas the Eheim flowrate reduces much more slowly as the filter clogs up with particulates. YMMV, but these are my personal findings.
As for your nitrite spikes, are you cleaning both HOB's at once? If so, don't. Clean on alternate weeks. If you clean both at once, you are removing too much beneficial bacteria. Also, are you cleaning the media with dechlorinated water? If not, that would kill all most of your bacteria.