I'm a bit biased because I've got the Sprenger ones which hubby bought me for my birthday last year - and I think they are fab. I really notice it now when I ride in non Sprenger or non flexi stirrups I can tell you!
The sprengers are safer than the cheaper flexi stirrups. Apparently something to do with the sprengers flexing backwards, forwards and side to side. The flexi ones only go forward and back and have been known to fold and trap riders feet in falls which can cause nasty accidents.
If you can go for the sprengers, a good cheaper option but still with all the different movement is the Thorowgood T6 iron.
There are some flexi stirrups that are very similar to sprenger but (i have both) actually have fewer links (like bicycle chain links- I looked!). Still do the same job. Then there are some other ones that have a sort of swivel tread, which are not so good.
Then there are some other ones that not only bend, but are also shock-absorbing, but I forget what they're called. Starts with an M I think. They're supposed to be very good.
I have the cheaper pippa funnell version, never had chance to compare.
Quote <font color="blue"> 'I love my Sprengers, but not tried any others to compare - after shelling out £90 I don't want to know the cheaper ones are just as good!' </font>
I was intent on purchasing a pair of the sprenger single-jointed (the first design that came out years ago), due to knee pain with jumping length stirrups (old skiing injuries). However, I had no intentions of paying almost $200 (US) for a pair of irons. I found some knock-offs made in Canada (Greenwood?) that looked similar, and purchased them for $30 (US), and have to say, my knees feel MUCH better in the saddle. So, I don't know if they're as "safe" as the HS design, but for my purpose of relieving knee pain, they work just dandy.