There are many many places where you can find so many threads regarding this issue. Somewhere in Amex forum somebody compared Amex spg with DC very nicely and I am copying that for your info as follows. The rest you will find tons of info in research.

the strengths and weaknesses of the Starwood American Express versus Diners Club Charge Card:

Starwood advantages...
Lower annual fee ($35 vs $95)
Free Additional cards (vs $30)
More airline miles per dollar charged ($20,000 = 20,000 points = 25,000 airline miles vs $20,000 = 20,000 points = 18,000 airline miles net after 95 per 1000 transfer fee.)
Annual 50% discount from rack rate on one stay of up to five nights
Annual $100 credit if book $2,000 "vacation" or cruise.
"Best Value Guarantee"
"Return Protection"
Lower foreign currency surcharge (2% vs 3%, I think)

Diners Club advantages...
Wider acceptance, especially outside North America
No preset credit limit
Payment due 31 days after statement date (vs 20 days)
Primary auto rental loss coverage (vs secondary)
Greater travel accident insurance benefit ($350,000 vs $100,000)
Baggage insurance
Purchase Protection $10,000 maximum per item (vs $1,000)
International airport lounge admission
Concierge service
Points can be transferred to lodging programs other than Starwood