Because of your timetable, I would strongly vote for the Peloponnese .... and I say this as a HUGE fan of the Cyclades: those are the 2 areas I have never missed on any of my 12 visits -- but then, I'm retired, usually go May-june or Sept, at least 3 weeks and on 9 trips have taken along 3-4 'travel pals' who are newcomers, so I need to show them my fave places in each area. Some plusses & minuses to consider in making your decision (you have plenty of time, don't rush)
• CYCLADES - Plusses: yes, so many gorgeous romantic venues, beaches, sunsets, no denying! Possible minuses: (1) in my sept. experience, there's an end-of-season feeling, some beach areas closing up, Greeks getting ready to live their own lives (3) after 4 rainless months, dry islands look more "brownish" (2) less ferry service (3) fewer historic sites/ ruins (4) island-hopping, tho fun, takes more time than Pelops self-driving & is less flexible; changing even to a nearby island can take most of a day.
• PELOPONNESE - Plusses: (1) Unlike islands, not as much season-end feeling, in fact beautiful Nafplio absolutely buzzes in Sept. w. Greeks & Europeans who love a quick fall getaway..
(2) Romantic views -- NAFPLIO!! Greeks consider it most beautiful Old Town in All of Greece with stunning site -- on tip of peninsula, surrounded by water, heritage-worthy golden venetian architecture. In fact known as Proposal capital of Greece because so many men take sweethearts there for the Big Ask. Check this wonderful NONcommercial website http://www.visitnafplio.com for town & entire area - check ALL links including up-top for directions, must-sees, where stay/dine/shop etc. PLUS, i consider Nafplio sunsets, seen from comfy loveseats in seafront cafes, to outdo much-promoted Santorini's any time. For a Quick sample of Nafplio plus famous area sites: https://www.flickr.com/photos/stanbr54/sets/72157632094108982/
(3) History -- The Winner!! Argolid area has more history per square foot than anywhere else in Greece, even in a quick swim in a little cove, I discovered Iliad history, all on my own! That's just the start -- think Mystras, Messine, Olympia .. and more.
(4) Beaches -- around Nafplio area, at least 4-5 fine beaches -- and since u have 7 days, I can pass on my own Secret Eden on West Coast, n. of Pylos... Hotel Zoe in Gialova... tiny beach haven, amazing setting, great nature hikes & history in area.
Possible Minus -- No Island/Ferry experience .. That's easily fixed! On Return to Athens, just get to Piraeus and do a day-trip (or an overnight) on a weekday, to nearby island - Aegina. The 1-hour voyage (on a "regular" ferry w. open decks, rather than closed-in no-view hydrofoil) gives you a "mini-cruise" experience of lovely Saronic bay & all its watercraft, ranging from huge liners to tiny sailboats ... check this lovely album https://www.flickr.com/photos/36264706@N03/sets/72157621604646139/detail/