The Polk LSi series are their flagship models. The RTi series is just below the LSi in "ranking".
That said, the RTi's seem to be favorites amoung HT people, while the LSi's are better received by those who are musically inclined.
The best comparison between the RTi's and LSi's is that RTi's have a bright, forward sound that is quick and responsive, while the LSi's are fuller and sweeter with greater detail.
Meaningless words, unless they describe the sound that you desire.
By the way, the Axiom's have been reviewed as being closer to the RTi's in style and sound.
Personally, my suggestion is buy what you like.