Orlando to Siesta Key will be probably 2 to 2 and a half hour drive.
SK is very close to Sarasota , so won't take you more than 10-15 minutes I wouldn't have thought.
Sarasota to Clearwater about an hour.
You are better off sticking to malls for your shopping, I have checked on my 'mall list'

for the area and best one's are below....
Westfield Mall Sarasota:
8201 South Tamiami Trail, Sarasota, FL 34238, 941-926-4890, The largest shopping mall within a 40 mile radius of Tampa, this exciting mall is anchored by JC Penney, Sears, Dillard's, and Macy's plus features over 140 specialty stores, a food court, and an AMC movie theater
DeSoto Square mall Bradenton: (half way between Sarasota and Clearwater).
303 301 Blvd W
Bradenton, FL 34205
The mall is located at the intersection of U.S. 301 West and U.S. 41 in Bradenton.
offers a wide variety of shopping experiences with some of the favorites stores, such as JCPenney's, Sears, Dillards, and Macy's. DeSoto Mall also offers over 100 specialty stores for your added convenience and shopping pleasure. The Mall features a wide variety of food court selections to satisfy anyone's tastebuds.
Tampa International Plaza:
International Plaza & Bay Street
2223 N. West Shore Blvd.
Tampa, FL 33607
813-342-3780
International Plaza, adjacent to Tampa International Airport, is the most distinctive shopping & dining destination of Florida's West Coast. Four fashionable department stores, over 200 specialty shops and Bay Street at the Plaze...an open-air village of fine restaurants & small boutiques.
St Petes Tyrone Square Mall:
6901 22nd Avenue N.
St. Petersburg, FL 33710
727-347-3889
Tyrone Square is anchored by Macy's, Dillard's, JC Penney and Sears, & features a large Food Court, a 6-cinema movieplex & more than 170 specialty shops, including Ann Taylor Loft, Charlotte Russe, The
Disney Store, American Eagle Outfitters, Victoria's Secret and Victoria's Secret Beauty.
Clearwater Countryside Mall:
27001 U.S. Hwy. 19 N.
Clearwater, FL 33761
727-796-1070
Featuring Burdines, Dillards, JC Penney, Sears and more than 170 specialty shops. And since shopping should be fun, they also feature one of Florida's only in-mall ice skating rinks, full-service restaurants, a food court and movie theaters.
As for whether you are better off staying in Clearwater I would say yes, the beach is fabulous there - to be perfectly honest we went to Sarasota several years ago and found it a bit dull to be honest.
If you stay at Clearwater you still have St.Petes and Tampa to explore.