Thread moved to American Express forum because both the Starwood Preferred Guest credit card and the Premier Rewards Gold card are both issued by American Express.
Starpoints are not worth three times the value of Membership Rewards points, so it seldom makes sense to transfer from Membership Rewards (USA) to SPG at the unfavorable 3:1 ratio.
To assign a value to miles or points you need to do your own arithmetic. I find this is much more useful than reading someone else's conclusions. You need to see the math to judge if it makes sense for you. Start by thinking about a trip you would enjoy (or need) to take. Determine the costs in money. Determine the number of miles and points required to pay for some elements with rewards. Use this to estimate values.
AMEX should just keep it simple with 1:1 only transfers.
I understand point systems like Jet Blue and SPG are worth more than 1 cent each, but that shouldn't be the cardholder's problem.