i hesitate to post, because i really comment much on the sonic differences between each. i ran the portico 5012 for a few shows, but never the 148. my purpose in writing is to point out that the 5012 has 2 options in terms of sound, you can have regular or w/ the 'silk' button engaged. i would imagine that w/ the silk engaged, it sounds very similiar to the 148, and w/out, maybe more transparent. this is a bonus (IMO) as you could decide to use the silk or not depending on the sound in the venue or band being taped.
although you have adjustable gain on the 5012, i would bet you would probably keep this somewhere around +20 and then change the gain on your AD or recorder - i am not that smart, but it took me a while to figure out that going much higher on the 5012 brickwalled my AD.
anyway, i think the 5012 is a great piece of gear and the better choice, unless you can find a 148 for a good price. i wish i never sold my 5012. hope this helps. cheers, ingsy