>Ultimately my dilimma is this, if it really comes from SP, then Powering Down isn't a problem, because I make more in a week than I Power Down. 

In theory if you can make enough from posting then you would be able to make up for the lost vests from powering down.  

>But if it is from Vest, then I will probably stop for a while, because earning the same amount in Vests after a power down is getting more difficult with the decline in the cost of Steem.

I think you've hit on the issue in your post and have reminded me of something.  The amount of VESTS that are equivalent to 1 Steem and hence 1SP vary and so whilst SP power is expressed as VESTs on the blockchain it is not a direct fixed or  1:1 ratio.  There is a way to check it but I can't remember right now.

Perhaps someone who is more of an expert on the inner workings of Steem may be able to help and confirm this.