6s vs. 4s?

[jsr]

Instead of thinking of how much mA your setup consumes per time period, think about Watts. So instead of looking at mAh, think Wh and do your calculation that way. Here's an example:

5000mAh 4s pack gets you 15min runtime
5A * 14.8V = 74Wh
This means you're consuming about 74Wh (not really as the LVC cuts it off before that) in approx. 15min.

Now, to get the equivalent runtime with a 6s setup, you want the same Wh capacity:
74Wh / 22.2V = 3.3Ah = 3300mAh

So choosing a 3300mAh 6s pack will get you equivalent runtime as a 5000mAh 4s pack.

Theoretically, the lower kV setup provides more available torque, but because the lower kV motors have more internal resistance and build up rpms slower, the torque is applied a bit slower thus you don't feel that additional torque as quickly as the torque provided from the higher kV motor (which gives the smoother feel). Also, the higher kV motor should theoretically have more top end speed, but because all these motors are way overpowered for the vehicles they're run in, we're not using all the available torque and power from the motors and the difference on the top-end is dampened so they how quickly they reach that top speed is not as noticeable.

Higher voltage is better for heat, however there are other items that wear a bit faster (such as the ESC caps as their life is shortened with higher voltage, but operating life should still far exceed your usefulness of the ESC). For me, I get 2000-2300kV motors for considerable lower price than the lower kV motors from Neu, Neu-Castle (their 1800kV motor is nice), and Tekin, so I'm sticking with 4s until the lower cost 4s motors are no longer available or lower cost 5s and 6s motors are available. Plus, my other RCs are set up to run on 4s also, so the compatibility is useful for me. Otherwise, I would run 5s or 6s.
For those concerned with the price of the Neu-Castle 1600kV, it's a monster sized motor that IMO is too big for buggies and truggies (you'd be using a small percentage of it's potential, which is why it runs so cool). The Neu-Castle 1800kV 1512 motor is a nice option that's similarly priced to Tekins and would run well on 5s (6s is a bit too fast and you'll lose most of that runtime advantage).

Oh, forgot about your battery question...
Don't go by the ESC amp ratings...they're all BS. They usually go by the rated FET capability at 25C instead of derating for operating temps or design limitations.
The higher voltage you go, the lower current capability you "need". I say "need" because most people get more than they "need" because the higher discharge packs have lower resistances (which is why they can discharge more current), so they feel punchier. There's a difference between "need" and "want". You don't "need" a 150A discharge capability pack, but you may "want" one. Also, higher discharge packs will run a bit cooler (again, due to the lower internal resistance). It's up to you.

[kingdomracer]

It's always so nice when someone comes along and clears up the fog in front of me, thanks. Now if you will explain to me the formula that you used to know that a 5000 mah 4s runtime is 15min. I already own a bunch of Enerlands that make up 2 sets of 4s and 1 set of 6s and they're 6000mah and using you formula they will be just 400mah under so they'll do just fine and should give me the same run time as my TP 8000mah 4s. I tend to stay away from 6s due to my share of fried MM ESC's but I cured that problem with xtra caps so I'll probably run a couple of laps with 6s just to see what it does but plan to run 1800kv 5s 47/12 setup on my 8 2.0

[georgec]

Thanks jsr, excellent information. Helps to put it in perspective.