hello, im my humble opinion if you considered the 28mm smc-a why not get the M version, it will probably cost less and you'll save more $$ for that 15ltd next year
I say this, even though i don't know that 28mm lens i mentioned, i know the pentax smc-m 35mm f2.8, to tell the smc-m lens are really, REALLY well built.
the only diference is that it doesn't have an A setting, but if you can easily use stop down metering at a press of button like i do
i have the 18-55kit, the smc-m 50mm f1.4 and the smc-m 35mm f2.8, and the 35mil is the best sharpest lens i ever used
maybe it's a good copy, maybe it's because it's M.. i don't know i got lucky and it was only 25eur
Basicly, if i were in your place i'd do
28mm f2.8 SMC-M and save the money for a ltd 15mm later (unless you want to wait and see whats coming for full frame?....
)