The best itinerary for you depends on your priorities.

If you want to visit Cape Town and would like to be coddled, then Option 1 is the way to go. Both Londolozi and Singita (I've stayed at both, and they're equals in my book) will coddle you, and offer amazing dining and service. For Singita, I'd actually skip Lebombo/Sweni and head to Tswalu - it's awesome. What you won't get in any S. Africa reserve is the "out of Africa" experience that East Africa offers.

If game viewing is your priority, then I'd opt for East Africa - however, given that it's peak season you need to pick your location and lodges wisely.

One thing.... if you really want to experience Giraffe Manor (it's definitely worth doing - Giraffes are incredibly charismatic creatures), be aware that The Safari Collection reserves several Giraffe Manor rooms for guests that intend to stay at one of their other properties as well. Have your agent check on availability on the assumption that you'll stay at Sasaab, Solio or Sala's Camp. Sala's is the only one in the Mara, but it's in a heavily trafficked area BUT if you want to see the Wildebeest migration it's very well situated. My suggestion: Book 1 night at Giraffe Manor and 2 nights at Sala's. Spend one morning doing the hot air balloon flight - the take-off point is conveniently close by. And if the migration is near, you can also prioritize checking it out. And then get away from the crowds and spend for 3 or 4 nights at Mara Plains Camp in the neighboring Olare Motorogi Conservancy. It absolutely thumps with wildlife, and you can still go see the river crossings (roughly 1 hour away) if there's big time action along the Talek River (or Sand River or others nearby.)

If you don't need to do Giraffe Manor (plus one of the other Safari Collection properties), then Tanzania comes into play. No, Grumeti will not be the ideal location for the migration, and the resident big game will likely be quite sparse by August/September. You'll need to be in the Lamai Wedge/Kogatende region, and Lamai is the best lodge in the area. I'd still give the edge to a Masai Mara / Mara Conservancy combo for a first time visitor, however.

Cape Town + Singita and/or Londolozi (plus possibly Tswalu) will be a terrific vacation with great food and pampering all around.

The Mara region has no rival when it comes to wildlife viewing, and it's THE place to go for a first time safari goer that prioritizes big game in a "classic" African savannah setting.

You'll have an awesome time either way. It just depends on what's important to you.