Mainly try to get enough port area to not get port noise and then get the proper tuning. As for which port is better, the more square/circle a port becomes, the more efficient it is as in it'll do its job without seeing much port noise compared to lets say a long skinny port aka all those prefab boxes with 1.5 inch x 13 skinny ports. The way you know this is by surface area of a rectangular prism basically The more surface area means the more air friction means less efficiency.
A=2(wl+hl+hw)
lets say you have a length of 20 and lets shoot for a port area of lets say 64 sq inches
One port will have 4 width x 16 height A=2(4*20+16*20+4*16)
Surface Area = 928
one port will have 8 width x 8 height A=2(8*20+8*20+8*8)
Surface area = 768
So as you can see the more square a port becomes, the more efficient due to less wood surface area that air has to travel through meaning you can get by with a lot less port area than a long skinny port.
here's a calculator for you to play around.
https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=surface+area+of+a+rectangular+prism&*
How about a round circle port you ask?
simple same length 20 and same sq inches 64
area of a circle is pi x radius ^2 so to reverse that 64/ 3.14 = 20.3. square root of 20.3 = 4.51 inch diameter round port.
Now you find the surface area of a cylinder. which is A=2(3.14)rh+2(3.14)r2
Plug that in to here
https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=cylinder+surface+area&*
and you will see that with a 4.51 diameter round port, the
surface area = 694.54 which is less than the square port aka more efficient.
to answer question
#1 it actually depends on the subwoofer and vehicle its in. Every vehicle will sound different and require a different type of tuning. Some will play rock just fine tuned at 32hz. Others will require 40hz tuning so its best to test out the cabin gain of each vehicle to see what frequency you peak at.
question number 2, There is no "hidden chamber" ported box. There are 4th order bandpass and there are Aperiodic Bi-chamber (ABC) ported boxes.
4th order is a sealed and a ported combined good low end extension and good output if you have a higher ratio of ported to sealed only works with woofers that are good in sealed boxes aka Efficiency Bandwidth Product (EBP) of 55 or less here is a calculator
Efficiency Bandwidth Product Calculators - EBP, Speaker Resonance Frequency, Speaker Electrical
Aperiodic bi chamber boxes are meant to tame peaky woofers for a flatter response.
Yup gramps there's waaaay more to subwoofers and enclosures than you'll ever know. What you know only scratches the surface.
mostly sound the same but will help slightly with airflow Its always recommended. Gains are very very tiny but gains are still gains.