I bought two RE27 on separate deals. Both came from broadcasting studios that purchased them, decided they wanted the RE20 instead, and sold 'em to me.
My conversation with the seller indicates a substantial difference in tone between the two. He wanted all his studio mics the same (RE20), so I got a deal.
I use the RE27 strictly for live performance work. IMO, they are great for this, as no proximity effect. They have good output, and are pretty bright. They have a wide cardioid pattern, and are more prone to feedback than an Audix OMx series mic. The RE27 excels with amateur talent that insists on swallowing the mic, because it doesn't get all woofy.
I suspect the RE20 has a bit more bottom, and is certainly lower output. I'd like to own an RE20 just for saxes or brass.
Opinions are like, well, opinions. Everybody has one, and it may not be the same as yours.
The only way to tell for sure is A/B yours against an RE20 and see the difference. To be fair, you will have to match the volume levels exactly, or the ear will prefer the louder of the two.
I suspect my pair of RE27 has less bottom than the RE20. Depending on usage, this is good or not. For live vocals, the RE27 cuts through very well and is highly intelligible. The opposite end of the spectrum for live work is my Beyer M400 SoundStar II. It is very dark and phat sounding compared to the RE27. It does not cut nearly as well in a live PA environment.
The RE 27 is actually (I've been told) a reworked RE20 using neodymium magnets which give the response of the mic a brighter or a more extended top end. As with speakers or say eq'ing a track the perceived lack of one frequency may simply be an abundance or over abundance of another. I find that the 20 has no more or less bass than the 27 but the extended or brighter top end of the 27 draws the ear to it and therefore we perceive the 27 as having less bass. The overall feeling I get is the 20 sounds more like a dynamic mic (which it is) while the 27 shows more of a condenser quality about it with more air on the top end and a better depiction of nuance. I've used both live and in recordings and while I can Q the 27 to sound like the 20 I can not do the reverse as the 20 simply does not have the extention.