Here is a list, based on my experiences, of what I feel are the best sounding AM transmitters (my opinions of course). The ones at the top of the list, while not sounding as good as FM mono, actually sound pretty decent (much better than I expected, as good or better, in fact, than most commercial AM stations).
1. Hamilton Rangemaster - the priciest of the lot, but the best sounding, particularly with an Inovonics 222 and Symetrix 421 before it
2. ChezRadio ProCaster - sounds almost as good as the Rangemaster with the built-in audio processing, at a bit lower price, and with easier installation and tune up
3. Talking Sign - a few cuts below the first 2. Sounds OK, inexpensive and uses a wire antenna so OK indoors (you can also add an external 50 ohm antenna). Discontinued.
4. Talking Sign - the earlier ones did not sound good at all. The IAM ones sound better, but still a cut below the Talking Sign.
I don't have experience with the SSTran products, but from what I've read, they can compare with the best.
For myself, I have no problems recommending either the Rangemaster or ProCaster. You can pick them up more reasonably priced used (anywhere between $200-400). The ProCaster has the added advantage that you do not require any external audio processing (although you can do it at the computer end with the Rangemaster without extra equipment). If you are concerned with sound quality, particularly after all the talk of the advantages of Part 15 FM, it would probably be better to spend the extra $ up front - it will save further down the line as you become dissatisfied with the results (unless you absolutely just don't have the financial wherewithall, of course).
As far as range goes, my experiences tally with the AM Transmitter Challenge conducted elsewhere (minus a few transmitters that I've never used). The Rangemaster had the best range by far, followed by the ProCaster, with the Talking Sign and then the Talking House following up the rear. You can increase the range of the Talking House with an external antenna or using the somewhat rare ATU.