That's a tough decision. I love the Paradigm Monitor series. I don't know if two 6.5" midbass drivers would be better than 1 8" driver. I would think the midrange would be better with the dual 6.5", but the lower bass better with the one 8". The Monitor 5 is basically the Monitor 7 without the internal volume. The 7 would probably double the bass output but at quite a cost. Without doing a side by side comparison, who can say? With the right sub, I would pay less attention to the 80Hz and under frequencies, and more on the mids and highs.
The Polk LSi series are amazing, but priced to reflect it. There's a few guys here on the forum that have them. I'm sure they'll chime in. Lots of honest reviews on the Polk website on the LSi's also.