The E609 is the cheaper version. There were complaints that it just didn't live up to the highly regarded MD409 that was discontinued in the '90s. So Sennheiser came out with the more expensive E906 which is KILLER, very, very close to the original 409. All of these in this series of their "flat" mics sound really good on guitar cab's. It's a warmer, thicker tone profile than Jerry's long-time Sennheiser 421. Trey's tone was famously mic'd with the 409 back in the 90's. Santana too. SRV too. The 409 (and maybe the 906 too) is my hands down favorite guitar amp mic, unless I'm truly going for a Jerry tone.
Personally, I just can't ever get a good Jerry tone with a SM57. It's just got the wrong peak frequency and lacks a certain midrange fullness, makes the Jerry tone a bit too harsh and thin. Sm57 on a JBL is a risky combination. It can and does work, but you have to be very careful to keep the mic's capsule safely away from the metal dustcap or else it can get way too metallic and painful.
I must say though, while the 409, 609, and 906 sound great and will work ok, it's a bit TOO warm and fluffy for that crisp, focused Jerry sound. Not bad, but not totally accurate either.
You just can't beat the Sennheiser 421 for Jerry. It really does have the right presence peak and tone profile. Clear and sparkly and strangely just NOT harsh. It's a larger diaphragm too, so it really helps give body and meat to that thin Jerry sound. I think that for anyone who has invested in a Jerry rig and is going for that sound, the Sennheiser 421 needs to be part of that rig. It's somewhat essential. And look at a lot of Dead photos to learn where Healy liked to place and aim it. That can make a lot of difference too. Not too close to the metal dustcap, but not too far either. Find that perfect blend of paper and metal and it seems that keeping the capsule just barely outside the outer ring of the dustcap (which is the voice coil too) helps get a good tone.
... and it's just like any other day that's ever been...