I will preface this by stated I am an HK addict, so I may be a bit biased. I bought a Vector 53 w/ a 16inch barrel. I later sold it. They cosmetically look nice, and are close to an HK in appearance, but don't really hold a candle to the real thing. Cosmetically, they do look pretty good, but I had a few issues with mine. Reliability was not 100%. It was decent, but it would hiccup every 100 rounds or so. I also did not like the fact the the inside if the receiver has no finish to it. The outside is painted black and looks nice but the inside of mine was steel in the white. Furthermore, the receivers aren't 100% spec and putting a collapsible HK stock on it was a pain in the butt that was only accomplished with the help of a rubber mallet. Those were my main complaints.
As for comparing my HK93 to an AR, 93's are definitely heavier than AR's and I even think they recoil slightly more. Also, if you thought an AR got dirty from shooting, good luck with HK roller-lock weapons. They are dirty as pigs after shooting. Parts are also expensive.
With all that, I love my 93. Damn if they aren't one finely engineered rifle. As with most HK's it is 100% reliable and goes bang every time I pull the trigger. I can honestly say that HK is the only company whose product I have never had a malfunction with. Ever.
If you end up going for the HK, my advice would be to save up for the real thing. Vector's are what, $1300? I just saw a 93 on HKPRO for $1950 shipped with both A2 and A3 stock. The damn A3 stock costs about $300. When I am spending that much anyway, I'd rather spend a couple hundred bucks extra and get the real deal. Just my 2 cents.