clathium: the computer doesn't pick random cards from the deck, it selects something like 5 different ones and spins them. So the probability of winning an upgraded card is actually fairly high. At least 50% less likely than from FGs, but it's pretty often. I've fought <50 HBs and won 5 upgraded cards. that's 6,000+ electrum, which let me upgrade 4 cards. depending on your deck, HBs shouldn't take that long to kill, especially compared to FGs, which take forever with any deck that can hope to secure even a relatively high winning percentage. With a mono-aether or any similar rush/swarm deck with adequate shielding and/or healing, you'll do just fine.
I prefer HBs because I get a rare from every 5-10 Top50 games, which sells for 150-250 coins. So it takes about 30 or so games to get enough electrum for an upgrade. Just from my limited exerience, it seems that for the time invested, HBs give more electrum (and score, probably).