4ohm or 8ohm subs?

im gonna build a self contained unit with deck, amp, 2 8's & 2 5.25's.

ive found a pair of rockford 8's on cardomain.com & they come in 4 or 8 ohm.  what would be better for this application?  i intend to run them in seperate sealed boxes.  they are rated at 150rms & my amp is a 2 channel running 180rms @  4ohms or 425rms bridged to one channel @4ohms

so 2 4ohm subs, one to each channel, will put out 180w to each channel @4ohms correct?

i am building into ONE box:

a headunit

an amplifier

2 8" subs

2 5.25" speakers

& one powerpack to convert 110v to 12v.

i am then going to put it in my workshop, plug it in, & listen to bumpin tunes while i work on my next project.

http://www.layitlow.com/forums/index.php?showtopic=153046 

that should give you an idea.  except im not gonna use a battery, & im gonna have the speakers properly enclosed.

one more question.

im trying to keep the size fairly compact.  would it be alright to have the 5.25's fire into the same enclosure as the 8's?

ie. have one 8 & one 5.25 in the same sealed compartment?

i will be using this head unit

http://www.audiovox.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&storeId=10001&productid=13666&langId=-1

& this amplifier

http://www.audiovox.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&storeId=10001&productid=13682&langId=-1

& im looking at these subs, which come in either 4 or 8 ohms

http://www.cardomain.com/item/ROCP18S42PAK

in addition ill run 5.25's or 6.5's if i can fit them off of the deck.

if i got 2 4ohm subs & plugged one into the left channel & the other into the right channel, wouldnt that result in a 4 ohm load @ 180w?  the subs are rated for 150rms so thats pretty close isnt it?  or am i missing something?  i dont need to max out the power, i just want to match up the subs with the amps output.