With the type of riding you say you're doing, I'd stay with a 400. Both bikes will do the job, and I'm sure you'll be happy with both.
If you read the review that Kiwi Dave included the link to in his post, you'll see that what the reviewer wanted most was more power when coming out of a corner and heading up hill. Myself, I consider that a challenge to my riding skills to maintain my speed as well as I can through the corner, so I'm not relying solely on a larger engine to make up for my lack of skill. Seriously though, what would you gain from a 650 when commuting to work and riding in the city? Better performance at stop lights? Who cares. You can only do that so often before getting bored at it, and the 400 will walk away from most cars with no trouble at all.
I suggest you try at least sitting on both before committing in your mind for what you want. The older 400 like you have, and the newer model are two different bikes. The only thing similar is the name and general shape.
Some of the advantages to the 400 in my mind are:
1. Weight. Try moving the two bikes around on the dealership floor by yourself.
2. Economy. The 400 gets mid-60s gas mileage for most owners. Even pushing the bike hard at a real 75-77 mph and bucking a strong quartering wind riding through Ellensburg and over Snoqualmie Pass, I still got 68 mpg.
3. Power. How much do you need? The 400 is rated to reach 105 mph. The 650 is rated to reach 115. Both bikes have enough speed to get you killed, and if you're caught by the police, you can easily get a "performance award" or have the bike impounded. Again...how much do you need?
4. Storage. The 400 has more than any other maxi-scooter in the world.
5. Handling. Both are fun to ride in the corners. Some will prefer one over the other. I've ridden with 650 owners, and I had to wait for them in the corners and I didn't feel like I was pushing the bike any.
6. Maintenance. The 400 has a CVT belt to replace every 20,000 miles or so. Read through the 650 section of the forum on the number of CVT issues that are coming up. While the percentage is small...if you get it while out of warranty, it is the cost of a 400 to fix. The CVT system on the 400 is so simple, there's little to go wrong. If it does, it won't cost much.
7. Maintenance again. Tires cost half what they do for the 650...and last twice as long.
8. Maintenance one more time. You have a very simple, but effective design on the 400. One cylinder. I don't know what they call it on fuel injection systems, but on the carb systems you had to adjust both carbs to keep the two cylinders balanced. I'm sure it still needs to be done. I don't know what it is called on fuel injected systems...because I don't have to worry about it.
9. The seat. Many people want to change the 650's seat. Few change it on the 400.
10. Looks. The 400 is svelte. The 650's nickname is "Lardy".
I'm sure a 650 owner will come up with a similar list. And hopefully, many more 400 owners will chime in too.
The bottom line to me, is which one "fits" you the best when you sit on it. And which excites you when you think about riding it? My 400 does both for me. It may not for you. Only you can make that determination.