I think the problem is that the devices out currently are just gap fillers for the real bad boys that will get released towards September and afterwards. You do not have to wait forever. Just wait until then. It will also drive down the current flagship prices to more affordable prices.
iPhone 5 will be out soon. And that will send all the other vendors into a frenzy. Just wait till end September. You will be able to make a sound decision then. If you want one of the current phones go for it. They are great phones. I am going to wait as the new devices shadow the current ones by far.
I am not sure why you see the HTC Sensation and Samsung Galaxy SII as gap fillers. The SGSII was released with all the new hardware Samsung could muster into a handset, and with it (if you do some reading) they basically gambled. They were worried about too many new things that could cause problems in the phone but the gamble played off. The only real improvement that can be made on the handset is probably a faster processor, and maybe a slightly higher resolution screen. Apart from that it has excellent battery life compared to competitors, all the interconnection features you could want, out-of-the-class processing power, and amazing visibility on the screen.
The HTC Sensation tried to compete and did pretty well although it lacks some minor features such as NFC, clear screen, battery life and processing power. But all in all it was not too far off from the rest. for Motorola and LG, yes their cellphones is not currently challenging the high-end phones from Samsung and HTC.
If you are maybe looking at 3D features in your phone (which imo is waist of money and time) then you could wait for newer Android device but don't expect Apple to follow suit - they are probably with the majority of us believing that 3D is a niche which does not add real value and not fitting in with their general cellphone "which is just what the user needs, nothing more and nothing less".
Another thing I would have like is a 4" screen with the SGSII performance but their is currently nothing on the cards or any rumours of phones that will fit this in the performance segment I am looking at.
Your valid argument is that the iPhone may be upgrade with to the GS or maybe even the 5 (although rumours indicated 5 to be 2012). What might you get that is better than the current iPhone? Better processing, more storage, maybe a bigger screen? Will it compare to the SGSII, probably not but if you are a fan of iOS it can be worth your while to wait for the September release, and then wait another 3-5 months to become readily available in SA if you are lucky. So that is a 7-9 months wait for your upgrade...
So in summary, if you are considering iPhone path and happy with your current handset, continue with contract on month-2-month and wait till September to decide if you want to wait till end of 2011/beginning 2012. If money is not the problem the SGSII and Sensation is pretty darn good and definitely not gap-fillers. If money is an issue then waiting will always help to get the current "super-phones" for more affordible prices, but then there will always be better phones coming out. And currently the HTC Desire HD, HTC Desire and Samsung Galaxy S are still pretty awesome phones which is now coming in on contract at R135pm.
For me I have to wait till September to do my upgrade, and unless the in-probable happens and a new Android phone is announced, released and available in SA before November I will be upgrading. Not a iPhone fan personally and will definitely not consider the WinMobi7 within next 2 years while it tries to get into the market.