Both sailing areas correspond to your desire for secluded anchorages. In April-May and October though, it will be more lonely than optimum, more crowded in July-August and about right in June and September. Alone means risking not finding tavernas open and serving food on shore! These areas present basically the same weather characteristics. In the summer, unpredictable and varying force (up to 6 B) trade winds (NE for the Sporades, W for the Turkish coast). In spring and autumn be ready to encounter strong rain-bearing southerly winds (very dangerous!).
In the case of Sporades, most cruises start from Skiathos. You can then sail (and relax) along the following itinerary:
Koukounaries beach (Skiathos island)
Paleo Trikeri - Milina (east coast of Pagasitikos Gulf)
Pigadi (at Pteleou bay, across in Pagasitikos Gulf)
Panormos bay (Skopelos island)
Patitiri (Alonisos island)
Steni Vala (Alonisos island)
Planitis bay (Pelagos island)
Linaria (Skyros island)
Ayios Fokas bay (Skyros island)
Skantzoura island (overnight stop)
Skopelos town port
Return to Skiathos town port
This is about right for a two-week charter. I suggest you charter a boat over 40 foot long. One last question: why don't you ask a quote from a Greek chartering company? To give you an example, I booked (early enough) a SO 43 for 4.500 euro, for two weeks during the second half of July 2006, through the following website:
http://www.fyly.gr/
Fair winds and
Please report your experience when you come back from the cruise!