The new suites are visually spectacular. Really well done aesthetically and they feel very special when you get in there. The crews are understandably excited about them and my perception is those crews have upped their already excellent service standards. Or possibly SQ just staffs those flights with their very best crews.
In the past year, I've done 10 or so flights in the old suites and 3 flights in the new. Only listing that so you know it's first hand experience.
Despite the new suites being more visually appealing, I personally don't find them an improvement functionally. Here are my complaints.
1. I do not like the seat. It's gorgeous and the leather is excellent. I just don't find it comfortable. It doesn't recline enough and the footrest isn't long enough. The latter is unforgivable to me b/c then your lower legs are dangling and that's very fatiguing on a longhaul flight.
2. The bed is slightly more narrow on the new suite compared to the old. Yes, you can leave it out all the time and still have a chair. So if you're someone that goes back and forth between bed and chair, then you may prefer this setup.
Take everything I say with the following caveat. My preferred seating position in the old suites is make the bed up for most of flight. I spend the entire flight in the provided PJ's. I work with my laptop in my lap. I watch TV like I'm in bed at home. And for meals, they have a temporary table that they can set up that attaches to the side of the bed. The last one is the only slightly problematic one and I will stay in chair mode if I'm eating immediately after takeoff or right before landing. But to everyone that will say it's a pain to make the bed and unmake the bed, I agree completely. Which is why I stay in bed mode.
But even without being in bed mode, I find the old suite chair more comfortable than the new suites chair. And I find the old suites bed more comfortable than the new suites. Oh yes, I should also point out that with the new suites, there is no way to sit up in bed. You'd be leaning against the inverted curvature of the windows. There is a way to prop the head of the bed up a little (like a hospital bed) to watch TV. But it's not comfortable and wouldn't work to say, sit up and work on your laptop.
In answer to your specific questions. Double bed is an option on old suites and you have a better chance with other single travelers more likely to take the windows. With 12 seats in old suites, you also have a better chance of getting 2 suites. One for a seat and the other set up as a bed. That's what usually happens for me.
Food and service are largely identical. As noted, service does seem slightly elevated and probably should be with only 6 seats to serve.
Definitely not a confined space. I find the old suites to be extremely generous. Better than just about anyone out there compared to ANA, JL, AF, BA. The new suites are just a ridiculous amount of room. When you can walk around inside your suite on a plane, that's not confined at all! That said, you are correct about the window. Dining and watching TV are both with the seat turned away from the window. And the chair turns very slowly so not practical to keep swiveling it around.
If I were you, I would just take whichever is more convenient. Absolutely no reason to take a positioning flight unless you just want to try out the new suites product.
Final comment. If this were a conversation between suites and standard F on the 777, I would absolutely tell you to take a positioning flight to get onto the A380 suites. The standard F is perfectly fine compared to other F. But the bed mode in particularly is not comfortable. There is a ridiculous flap over that connects the seat to the footrest and it's not the full width and it doesn't sit flush. So you will wake up with a sore spot wherever your leg hits the hump. FRA has one flight a day on each. I believe LHR is 2 of each. Make sure you pick the correct flight. F is the same price as Suites so take Suites.