They're VERY different! I assume you are in the market. If you are, the fact that you're even curious would make me tell you to go out and do some listening to find out what your tastes are. The A3 is very neutral, and the most "solid state" sounding amp Musical Fidelity has made. Their previous generations have tended to be "warm and tubey". The Golden Tube IS "tubey" sounding, mainly because it IS a tube amp. In fact, it is more classic sounding than many tube amps of today(which have been becoming more neutral). Today, many of the high end SS and tube amps are sounding more alike than they ever have. The Golden Tube bucks this trend. I, for one, am happy they do. I predict you'll prefer one over the other, most people do. The determining factor is you. The MF is more powerful, has tighter bass, and more extended highs. The GT cannot drive as difficult a speaker; has the looser bass, glowing midrange, and sweeter highs. Reliability would definitely favor MF, soul GT. If you're a guy who feeds himself a steady diet of hard or classic rock, you probably like the MF. Jazz, folk, classical, or a mix, the GT. GT has really dropped of the face of the earth in the last 2 years, so if you like the tube sound, you may want to consider another brand(Jolida lower, Rogue higher). But, I think your main question is to find where your tastes lie.
