Originally posted by eaglem sigma 10-20mm F2.8
mmm i think you mean f3.5 or f4-5.6...
There are some UWA out there that you may consider Tamron 10-24,pentax 12-24,the sigmas...or you can go for prime quality with the 15 f4 limited or the different 14mm (there is a pentax, a samyang, an SP line Tamron).
Zooms are more versatile and some of them have got great Image Quality, primes usually got better IQ and are hadled differently, they may make you think more about the composition and force you to move to discover the best framing.
If i were you i would shoot for some time with the two lenses you already got and then i would define the things i like the most, then i would buy the best lens that i could afford that suited those interests.
Since you already know you like to shoot architecture i would start to look at the Ultra Wide Angle lenses...For the indoor shots, if its shots of rooms and spaces i would look into a good tripod too to use with the ,usually, quite slow UWAs. If it's shots of people or things indoors with available light i would start looking at 24 to 50 range of fast primes or at the 17-50 constant f2.8 lenses.