Skim through the HI thread, it's been discussed there many times, there is no better or worse, there are subtle differences depending on what you are looking for in your situation... It is literally the size of the log, and as a result the size of the bowl, the diameter of the stems you will use as Raz said. Both are extremely capable and have a wide range of both sizes they can do, the larger size also means some larger mass for the heater (though I think the actual heating element of the steel cartridge is the same inside) and Alan is always innovating ways to improve that with all of the models. When you email him he can tell you more in detail about the differences and specificities of the designs, the new website has a lot of detail already though now... Furthermore before the different sizes were available, the glass core hybrid heater cover has been around even longer as an option compared to the standard steel core heater cover, those differences have been laid out in the thread many times by different people: the way the stems connect is a little different if you see with glass you cannot leave the HI upside down on a bowl you have to hold it there, steel core has a little rest ring. Also the type of stems are different, aside from the different sizes, there are different stems that work with the different cores, there are an insane amount of stem material and style variations that Alan makes... The power settings, what voltage level, are also different between all of them, different effects on the vapor quality, flavor and density, based on the different designs and materials, it is subtle and subjective, without trying them all you would have really no way of knowing any difference practically and even if you do have them all it's a subtle thing like I said, they are all extremely capable! Heat up time, and recovery time after a hit, also depends on the different size and materials between the different options... No wrong choice, I have the standard small glass core HIbrid that was my first custom and then I got the steel core next time my name came up, right now he's finishing up my large steel core custom, so then I will get a large glass core next time!
