911 Coupe. For many people the only true 911 is a coupe, for sure they have beautiful, classic lines, the shape of the roof is gorgeous and if you are going to use the car every day, come hell or high water than a coupe certainly makes sense.
If you want to use for track days, sprints, hillclimbs than the added strength of the metal roof will help reduce flex. But these are old cars and they flex a bit anyway. For serious driving a coupe with a good safety cage is the answer.
On the road a coupe is pretty quite and refined, the electric sunroof (if fitted) is nice and can be used effectively at most speeds.
911 Convertible. The convertible is actually a much better car than you might expect. The top is very high quality and seal really well. Many of the later (87 on) models will have one touch electric operation.
The top is generally reliable, fits well and can last a long time. I have owned many 15 year old 911 with the original canvas top, still in good condition. Of course your climate will make a difference as will the chemicals used to clean it (some rot the stitching). The rear screen is plastic, can be replaced separately but is brittle when cold. Good practise is to unzip the screen and lay it flat before lowering the roof.
On the road these convertibles are very refined for their age. Forget ideas of flapping plastic like an old MG, these roofs are lined and create minimal wind noise. If they leak much its either rubber seals that are damaged or badly adjusted roofs. Top tip is only use a company that are really experienced in 911 tops near your car. Make the effort to use the right people it will pay off (speak to your local Porsche club for a recommendation).
Body flex/shake and rattles are surprisingly minimal, in fact better than many more ordinary modern convertibles. They still handle well and feel nice. The downside is that the hood does not fold completely out of sight although each car should have a cover in the trunk to clip on and tidy up the look.
There are various products available that clean and seal/protect the canvas (try Autoglym) and others that help recolor the top.
911 Targa. Porsche manufactured a Targa top 911 right from the mid sixties and kept it in production until the end of the 964 series in 1993. Some people give the Targa bad press but it was popular and worked reasonably well. You simply turn two clips, lift the top off, fold it to about 2/3rds of the original size and store it in the trunk.
The advantage is that you have a car almost as secure as a coupe, and with better rollover protection than the convertible. With the roof off, the big glass rear screen offers wind protection (especially for children) and the possibility of lap and diagonal rear belts.
On the road it makes a nice ‘sunny day 911′ (which is the way many are now used). The disadvantage is that with the roof in place you tend to hear more wind noise from the top of the screen pillars and the odd dribble is inevitable.
If the roofs don’t fit well on your car, it will leak like a sieve and needs a really good (patient) man to sort out. If they fit well they are fine. Flex , rattle e.t.c is no difference to a convertible.