You asked a multi-faceted question, so answer accordingly.
1. Are Macs better as desktops or laptops? Hands down, Apple makes more competitve and stylish laptops. Now don't get me wrong, the iMac and Mini are still both great in design terms for desktops, but they have a few drawbacks. For example, the iMac and Mini both use laptop parts. Both of them are also a bit more than equally equipped PCs. However, the laptops are much better equipped and have excellent design. Compared to other laptops of similar power, the macbooks and macbook pros are lighter, thinner, and have better battery life. Granted, there are laptops that have more battery life, lighter (macbook sized esspecially), and maybe more power, but they never have the same blend of all of these attributes.
2. iMac or Mini? iMac. The iMac has much better specs and provides a much better future-proof machine. The Mini seems like a good deal because it's half the price, but it doesn't have a display, keyboard, or mouse. Factor these in, and the cost comes close to about $900. So think of it this way: do you want a six month old machine for $900 or a brand new machine for about $1200? The Mini is only good if you have the display, and your workload for it is going to be light. If you plan on anything serious, forget about the Mini; the iMac will be a much better servant for many more years.
3. Should you go Mac or PC? This really depends on your attitude towards Vista and HP. Personally, I don't like HP laptops. If I was ever forced to buy a PC laptop, it would be either a Sony or a Lenovo (IBM). These two companies live up to my standards (as does Apple). I'm not basing this on random opinion: I used to own an HP laptop, and I've had plenty of time on a Thinkpad and a Vaio. In order to make your decision, ask yourself if OS X is doing something for you that Windows can't. If OS X makes you more productive, wastes less of your time, generally makes you happier, etc, then maybe it would be wise to buy a Mac. If there's nothing you see in OS X, then you don't "need" a Mac per se.
Hope this helps
