The rule of thumb is that you'll want a bag either half your body weight (recommended for amateurs/beginners), or equal to your body weight or more (if you're a pro). And if you're rich and serious about training, you'll want to increase the size of the heavy bag and decrease the size of the speed bag every six months. Your best bet is to get a water-filled heavy bag. That way you can increase or decrease the official weight and firmness of it to your liking, and it's much more realistic to hitting a human than any other heavy bag. Also, if you need to store or transport it, you can simply drain it and refill it for later use.
But remember, if you want jacked up product and shipping costs, as well as customer service incompetence and complete corruption and rip-off tactics, by all means order from Everlast or Title. Their business is selling cheap (and often previously used), half-broken, Chinese-made products at 5x their value, while shipping it at additional unreasonable rates, while being governed or oversaught by no one with any moral or ethical values towards keeping the company name at what it once was. Some will say that only their lower-end stuff is garbage, but it's the same with even their higher-end stuff as well. Funniest of all, you'll order an item when it's on sale and they'll confirm that the item in question is indeed on sale, but they'll secretly bill you for the standard item price and other sneaky little corporate bullshit like that. It's sad, really. If anyone reading this doesn't believe me on anything I've said, please order from them and witness it for yourself.
Anyway, onward to the brands of boxing products. For boxing products in general, I would recommend Ringside, Ultimate Classic, and Combat Sports if you want a decent product for a decent price. Search them and you'll find their sites or sites that you can order their products from. Although of those three, I've only tried Ringside, I've heard good things about the others. If you're willing to shell out a lot of money for some really good gear on the high end of the market, get Cleto Reyes or Grant. Absolute top-notch gear, but very pricey.
But remember, nothing from Everlast or Title, ever. No matter how small or simple the order. Good luck.