I think the TSI's are supposed to be replacements for the monitor series. The TSi's have slightly better low end response and their finishes are nicer. But it's not going to be much of a sound difference from the monitors. If the monitors are half the price go with them. If it's only $20 a pair difference then go with the TSi's.
As far as the Polk products it goes R series(R50, R300), then Monitor/TSi, then RTi, then LSi.