There are indeed rare and champion mobs in Whimsyshire with plenty of chests to loot. I have ran the level a few times on hell and the loot is what you would expect. The loot is equivalent to that of act 3 or 4. The pro to Inferno is that it has the ability to drop better, higher end gear. If you can farm Inferno (at least the skeleton king) at a regular rate I would suggest that you do that. If Inferno is giving you a hard time, Whimsyshire is as good as any other place in Hell to farm.

So, any Demon Hunter Build is going to divide its skills into two basic categories. Tools You Use To Stay Alive, and Tools You Use To Kill Things. In general, these two categories are largely separable, with the former category being the one that changes radically in Inferno. I'll cover both here for completeness sake, but keep in mind that the latter section allows substantially more flexibility.

Staying Alive

There are two schools of thought for staying alive. The first is built on maximizing Smoke Screen uptime at all costs, through the use of the Lingering Fog rune, Preparation with either Focused Mind or Backup Plan (to taste). Multishot with Suppression Fire, as are the Nightstalker, Perfectionist, and Sharpshooter passive skills. These builds have become somewhat less popular after the most recent round of hot fixes, as the maximum possible uptime has been reduced considerably, but Smoke Screen is still an essential element of most builds, regardless. Since the most recent hot fix, you may also consider replacing it with Shadow Power with the Gloom rune. You'll experience a higher uptime of less damage reduction (-65%, rather than -100%). If you go the Shadow Power route, you'll probably want to focus on offensive skills that provide more health regen (see below for options there).

The other school of thought is to maximize your kiting ability. While Smoke Screen is still essential to most of these builds, they may forego some of the other +Discipline stacking options in favor of abilities like Caltrops and Vault, particularly in combination with the Tumble rune, which makes it near-spammable.

Either way, the key is to select skills that maximize your ability to not be hit. Whether you do this by increasing your invulnerability uptime, or by augmenting your mobility is up to you.

Killing Things

On this front, you have much more flexibility. Elemental Arrow is incredibly popular as a main attack. At a cost of only 10 Hatred, you can almost spam this exclusively in most encounters - Hatred builders aren't often needed, (especially if you use the Bat Companion, see below) which means that you can rely on this skill primarily for damage. The two most popular runes are going to be Frost Arrow, which offers high single target damage, decent AE, and a snare effect, and Nether Tentacles, which offers the best damage on single-target encounters, as well as a decent life-leech effect. Screaming Skull is favored by more kite-centric builds, as the fear effect offers a tremendous amount of crowd control - it's not as reliable as some other options, but it's potency and spammability more than makes up for it.

Rapid Fire is also quite popular, particularly with the High Velocity or Bombardment runes.

Hatred builders, by contrast, are much more to taste.

Evasive Fire is a very popular Hatred builder, but the risk of spending Discipline by vaulting unexpectedly is often enough to make it less useful than the alternatives - especially when action bar slots are scarce after stacking Defensive skills. Additionally, the damage isn't terribly high, and it offers a minimal AoE coverage or additional utility.

Entangling Shot is probably the most popular alternative, as the slow is handy, though the damage is pretty lackluster. Bola Shot and Hungering Arrow have proponents as well. This slot on your bars in particular, is largely going to be to taste.

And Then What?

At this point, you have a Hatred-Spending attack, a Hatred-Generating Attack, and probably 2-3 skills allocated to defense. (Smoke Screen or Shadow Power, and some combination of Prep, Vault and Caltrops). You can fit 1-2 more skills on your bar. What do you take? There are a few major contenders:

Companion, probably with the Bat rune - this is an easy attention-free source of additional damage. More importantly however, by generating 3-hatred per second, it allows you to use Elemental Arrow 5 more times (or get 5 more ticks of Rapid Fire), give or take, before running out of Hatred. That's a Really Big Deal, and a lot of extra damage, before even factoring in the 45% Weapon Damage that the bat does on it's own.

Marked for Death, particularly with the Death Toll rune is popular for Shadow Power and Nether Tentacles based builds for maximizing health regen. It's less useful if you're doing a lot of kiting, or concerned with AE damage. It's also very potent in combination with Nether Tentacles -- especially in 'true' single target (i.e. only one thing on the screen to hit) situations.

Multishot with Suppression Fire, as mentioned above, is a popular option for maximizing Discipline Regen. Alternately, with the Fire At Will rune, it becomes a popular AE option for builds that are more focused on single target damage with other skill choices (i.e. (Rapid Fire)

Spike Trap, particularly with the Scatter rune is popular for kite-centric builds - dropping one at a tight choke point can be devastating.

Passives

The only really important one here is Archery, which, in combination with a 2H Bow or Crossbow, is a massive increase in damage. If you're stuck using a Hand Crossbow, this is less important, but you should really be trying to get your hands on a two handed ranged weapon as soon as possible in order to use this passive. It provides an absolutely staggering amount of additional DPS.

Beyond that, it's largely to taste. If you're focused on SS/SP uptime, you'll want some combination of Night Stalker (possibly with Sharpshooter to support it), Perfectionist, and Vengeance. A kiting focused build may want Cull the Weak or Tactical Advantage. You have a lot of flexibility here.