It absolutely does not matter what one you decide on as far as sagging in concerned.
When installed properly, it can not happen!
I have intalled hundreds of squares of vinyl and my share of aluminum (for my age), and vinyl wins hands down.
You said that you are "having" your home sided, so I am assuming that you are useing a established, insured, licensed (where required)home improvement contractor. I would stay away from the types of contractors that also sell toasters, socks or 2 X 4s.
Ask for a copy of the liability insurance to be sent to you from the insurance agent, before you part with any green!
Make sure that you have your home improvement contractor use a solid soffit panel. That is one that is not co extruded or what some call grey backed.
In all fairness to your contractor, you should pick the brand and style out with him/her before hand and have it in the contract.
I have vinyl siding (Wolverine Benchmark) on my home and I used vinyl soffit and aluminum fascia.
I prefer the vinyl soffit because the solid color panel holds up better. All the bees nest and spiders and such tend to attach themselves to the soffit panels. When you scrub the soffit, if it is aluminum, or co extruded, you will, over time wear off the finish.
You said that you are useing both plain and vented soffit. Make sure that, if you don't already have it, you or the contractor cut vents in the wood soffit, between every joist prior to installing anything over it.
The reason I prefer aluminum for the fascia, is because you can bend the aluminum to fit any fascia. Also, if your roof does not have a drip edge on the peaks, you can (and make sure that this is going to be done if needed,)bend the aluminum to go under the roof shingles for flashing.
Now for some unsolicited advice,
Depending on what you have for siding on your home, I would consider removing it and installing high density styrofoam insulation like a 1/2" NRG or Energy Shield brand for example. They come in 4' X 8' sheets and won't build your walls out that much so your windows won't look like they are in tunnels.
The cost of removing your siding and installing the insulation should be a non factor.
On a 30' X 60' Ranch, you probably have 12 standard windows, 1 picture window with 2 flankers, 2 doors and a garage door and a 6' roof pitch. So you probably have about an 18 square job. Depending on your area, it couldn't cost that much and the job will look so much better.
Alcoa/Mastic is a very good brand. Just make sure that it is a solid color all the way through. I remember a style that had a photo finish applied to it, it was fine as long as you had no trees or shrubs next to it.