Wait and see means you give up $3750 in tax incentives, assuming you qualify for the full $7500 deduction. There's also the possibility that the tax deduction is completely eliminated in the next budget, so I guess it comes down to how long you want to wait, vs. how likely you think it is you'll get a partial tax credit in 6-9 months time, vs getting $7500 in tax credit today with an MR battery vs "saving" $5,000 on an SR battery but losing $3750 of tax incentives.
In short, MR today is a $1250 premium over hypothetical SR of next year. If the SR is delivered in Q3/Q4 the difference is pretty much wiped out and today's MR is actually LESS EXPENSIVE than Q3/4 2019 SR (tax credit down to $1875 2nd half 2019).
It's amazing to me people don't see this. Anyone waiting on an SR who knows they want PUP and qualifies for the tax credit is a dumbass if they are not buying an MR car today. About the only reason I can see waiting is you want APV3 hardware, Tesla continues improving quality, etc.
As far as what Tesla will cut for non PUP models we already know some of it;
1. No power folding mirrors
2. Metal roof
3. "non premium" materials interior, probably meaning cloth seats.
4. Manual seats vs power seats
5. no center console organization... possibly no armrest.
6. Lower end stereo.
7. Various other amenities such as USB ports for charging phones, etc.
One can hypothesize about lots of other things Tesla will do to deliver $35K car. I think if they want to continue to be regarded as luxury brand they will be hobbled in how much further they can go. Base Audi A4 or BMW 3 series are still decent cars even if they have cheapo leatherette interiors.