Is there any real difference in final file size between 256k audio and lower (most DVDs seem to be encoded to 192k) and variable bit two pass rate for the video?
If you are playing back content on a platform that decodes (or will passthrough) Dolby Digital then there is no need to transcode it - so you can do an "-acodec copy" and leave it untouched.
However if you are taking a 192k Dolby Digital (aka AC3) 2.0 source and transcoding it, you want to avoid concatenation compression artefacts (i.e. the Dolby Digital psychoacoustic encoding interacting with the different AAC psychoacoustic.) As a result I usually bump up the AAC bitrate a bit to try to avoid introducing needless compression artefacts. You could go a bit lower if you were starting with a cleaner, lossless compressed (or uncompressed) source like WAV, FLAC, Dolby True HD, DTS HD MA etc. However for the sake of 64kbps it seems a bit pointless. If you can't hear the difference, go for your life !
I know it really depends on the source material and how complicated it is but the general consensus from everything I've read online seems to suggest the variable bit rate generates a better picture but will obviously take longer to render all other things being equal.
Variable bitrate will give a better picture for a given average bitrate, than constant bitrate at the same bitrate - and thus eventual file size (everything else being equal), and two-pass encoding will optimise quality for a given bitrate yet further, at the expense of encoding speed. However if you use -crf encoding you're chosing to use a variable bitrate to achieve a constant quality, at the expense of fixing the bitrate and file size. However the result is also faster. For me it offers the best compromise between speed and quality - as storage is of relatively low importance to me (so I don't mind if the files are a bit bigger)