There are two types of extenders: spring-loaded extenders and non-spring-loaded extenders. Virtually all the extenders on the market are the former. They all provide the exact same function so you will yeild the same results no matter how much you spend on an extender.
There are only two things, in my opinion, that matter when it comes to purchasing a spring-loaded extender:
1. How it attaches to your glans. Most cheap extenders will use a noose style attachment, but even some expensive ones such as the Andropenis also use it too. The noose style attachment is by far the worst method of attaching your glans to the extender. Another style of attachment is the strap, however, this is not much better than the noose. Both of these will require a lot padding to be somewhat bearable when stretching over a moderate tension. Some of the more expensive spring-loaded extenders use a vacuum attachment such as the PeniMasterPro. This is a much better style of attachment, as it will not restrict the circulation of blood to your glans as much as a noose or strap style attachment, but please not that there will still be some blood restriction and you will still have to monitor for such. Also, it is really worth noting that you can buy a VacExtender mod kit from AutoExtender that can attach to most cheap extenders (although you should e-mail MB with the specs of your extender for confirmation), so shelling out $200-$300 for an extender with such an attachment isn’t the only option and you can also save yourself some money. Finally, there is the VLC-tugger which utilises the foreskin to provide suction on the glans. So far it is the method that will least traumatise the glans when attached to the extender. However, as far as I am aware, this only works if you are uncircumcised.
2. Spring strength. All spring-loaded extenders, as you would imagine, come with springs installed in the base rods. The strength (or ‘compression force’) of the springs vary with each brand of extender (this is the only factor that differentiates the spring-loaded extender in terms of function) So when you’ve been wearing your extender for several weeks and through gradually adding rods you finally start to compress the springs, there will be varying levels of traction force depending on the strength of the springs and the more the springs compress, the higher the traction force. However, the strength of the springs do not matter too much, as long as you can determine the traction force - when you’re compressing the springs you want to stay at around 800(1.8lb) grams to a 1800(4lb) as to avoid injury. Saying that though, the level of traction you want to be aiming for, once you’ve reached compression of the springs, is an on-going debate.
I hope that might help you make your decision, good luck with your gains!