I would go ahead and just get the spot attachment. That way, you are always using the same meter. I prefer the Pentax V or Digital Spotmeter, but if I already had the one you have, I would get the attachment for sake of consistency. It is cheesier, but also cheaper, and like I said before, this will cause your one meter to lead you to the same middle grey tone regardless of whether you are using it to meter incident light, reflected light, spot reflected light, or flash.
I like the split scale of the 1/21. However, it has a battery issue, as of last I used one (borrowed from my school several years ago). The battery for either the low scale or the high scale is no longer made. I forget which scale it is, but am thinking it is the low scale.
As for analog vs. digital Pentax Spotmeters, I prefer the way the analog ones read in the viewfinder (higher precision due to infinite "in-between" positions of the needle, while with the digital, you only have 1/3 stop precision). I prefer the weight of the digital. The digital is also smaller, so fits more easily in smaller pockets than you would need for the V. The exposure/EV/EI scale on the digital is quicker and easier to operate (though your settings can get accidentally changed a lot more easily because of this). Also, the digitals can be bought new, while you are limited to used meters with the analog. Both seem to hold decent value on the used market. I think that the digital can meter one EV lower than the analog. It may seem that the digital is not as solid, but after using both, I think it is actually the analog that is less solid. It seems like the casing of the analog is a hard plastic, while it is a lightweight metal on the digital. My digital meter fared very well in two accidents that I though would have fried the meter (one mechanically and one electronically).