The Classic is more than adequate. It's maximum resistance level of 90KG offers a very intensive workout. The Green Giant is capable of double the resistance level @ 180KG Max. The good thing about all the Isokinator's is that the user can set the device to any resistance level lower than the maximum, so there isn't any pressure on the user to immediately start working out at some ridiculous resistance level. It's completely adjustable. Want to train at just 10KG or 20KG, that's fine. It's a gradual process to work your way up to a higher resistance as time goes by and you become more efficient at the exercises.
That's why, technically, a total beginner could buy the Green Giant and get a long just fine with it. Don't be put off by the higher maximum resistance level, because the user never has to train at that pressure level if they don't want to. Truth is, the only real difference between the Classic Model and Green Giant, other than the slight differences in the design and the price, is the maximum resistance level. This makes the Classic model much more suitable and affordable for beginners.
The Green Giant is really for more advanced users. The individuals who have done their time using the Classic, have conquered the full 90 Kilograms and are ready to take the next step by increasing the intensity even further by upgrading to the "Beast". These individuals also know, from owning the Classic, that they like what the device offers, are dedicated with the training routines, and will get their moneys worth from many years of continuous usage.
Someone who's never used an Isokinator before might train with it a few times and just not like it. So its best for the novice to start with the cheapest model to begin with and see if they get along well with the device and the exercises. Watch some of the online training videos to see what the device offers before committing to purchase. Isokinator's resale value holds very well, so if you do decide to sell yours as used on a website like eBay, it will fetch a decent price. They rarely come up for sale second-hand as people tend to hold on to them.
So, is the Green Giant worth purchasing?
Well, in terms of quality, and excellent customer reviews, it's definitely worth it. Build quality is superb throughout the whole range. But more importantly, on the matter of whether its worth it for training purposes, as mentioned above, for experienced users who are now finding the Classic's highest resistance level no problem to work with, then yes, it is worth upgrading if you are a dedicated Isokinatorist. For the beginner, the Classic model makes so much more sense.
The only downfall, however, is that if the beginner does buy the Classic and ends up really liking it, they more than likely will, at some point, want to upgrade to the Green Giant. So ultimately they will end up buying two Isokinator's. Whereas, if they just went straight in for the Green Giant, they would save money in the long run. Because in doing that they then own just one Isokinator, which just so happens to be the biggest and baddest model. This way they never need to upgrade.