The weight of the keys won't affect the sound. It just affects your playing. It depends on what you are used to. So you need to play around with it to know.
Depending on your genre or what you are going to use it for, you may have needs for different types of controls. If you are into electronic music, knobs are going to be important. If you are more into production (multi-track, mixing, etc), faders are important (and you need to make sure you have enough of them). If you are into drum-pads, then trigger pads are important. If you are like me who rely heavily on cross-fading (one sound morphing/fading into another), then mod wheels are important (faders can do the trick too, but it's just my preference). Some use ribbon controllers for X-Y control; some use sticks for X-Y-Z control. Each of these controls lets you manipulate effects on the sounds differently - hence why I'm asking you about your genre of choice.
Also, if you are into orchestral (like me), you may want an 88-key controller. Many orchestral samples use key-switching (ie certain range of your keyboard is mapped such that hitting a key will switch a sample - important as orchestral instruments comes in various articulations that require quick swtiching; e.g. strings are played legato, arco, staccato, marcato, detache, spiccato, tremolo, pizzicato, col legno - one hit on the key and my entire string sample switches, enabling me to play the strings as it should sound in real life). In this case, you really want an 88-key controller as the key-switches mapping takes up a large range of your keyboard.
Also, faders/knobs/wheels etc are not all made the same. Some feel like it's going to break when you play too hard with them. Some faders are automated (and you pay for them!); most are not. Some knobs are "endless" (it just keep going round with no beginning or end) while others are not. It depends on your needs. And you really need to feel them to decide.
My suggestion is to stick with Novation. You can't really go wrong with it.