Best Saturn 5 kit

Neither kit is fully accurate. But if you go to John Duncan's site - http://www.apollosaturn.com/modeling/2005natscode/saturnvcode.htm  he takes you step by step thru the accurization.

Correct Real Space Models does have some aftermarket parts for the Apollo Block 2 conversion and the Skylab shroud. I think the F1 engines and a CSM are only available in 1:96 scale while the others are 1:144

But the engines should matter very much if you look at the engines during launch, they are insulation batted so all the details are covered anyway - http://apollosaturn.com/modeling/foilbatt/batt1sh.jpg

The Airfix Saturn V is still available thru Squadron, IIRC.  The Monogram kit isn't hard to find at swaps or on eBay, though.  Which kit is better?  It's a tough call.  Airfix has a lot more surface detail.  Monogram is somewhat toy-like but still good. 

Neither kit has very good J-2 engines on Stage 2 (although the single J-2 on stage 3 of both kits looks better, for some odd reason.)  Neither kit has a very good (in terms of both detail & accuracy) Block II Apollo CSM.  The Airfix CSM is unusable, as the capsule is narrower than the service module, and the boost protective cover is extremely thick!  Both CSM's represent the Block I configuration, according to people who can spot the differences between the Block I and Block II SM's.  Luckily, RealSpace makes replacements for the Stage 1 engines and capsule (they offer a Skylab too.)  Hopefully some replacement J-2's are on the way.

Be prepared to dig into your wallet if you want the Launch Umbilical Tower.  Here's a link.  It is available in 1/144 scale for now.

http://www.launchcomplexmodels.com/

I know Revell use to make a 1/144 Apollo Saturn V kit and Airfix did also but I don't know the quality of them.  I've built the Revell 1/96 kit and it is not bad even tough it lacks a lot of details and have some inaccuracies.  You can find upgrade kits for the Skylab version at http://www.realspacemodels.com/ and some detail sets at http://mek.kosmo.cz/newware/#Kits%20and%20detail%20sets

A full listing of all the plastic kits available on that subject can be found here: http://www.ninfinger.org/~sven/models/space_models.html

Hope this help.