Both Genestealers and Raveners are combat creatures with ability to engage the enemy early in the game. Raveners are beasts which gives them a potential charge range of 24" and gets them across the field very quickly. Genestealers are normal infantry with Fleet but can infiltrate which puts them up close to the opponent early, especially if you can hide them. Both utilise high weapon skill, initiative and number of attacks combined with Rending to deal with foes before they can retaliate. Neither of them are particularly durable but cover helps this a lot and the units are all about weakening the opponent in combat before they can swing. Like many Tyranids however, they suffer from lack of offensive grenades and whilst they can move through cover quickly, assaulting through cover for both of these units can be a death sentence. Let's look at the particulars then and their specific differences.
For every Ravener with Rending Claws you can take two Genestealers with Toxin sacs for nearly the exact same points (35 to 34). In terms of offensive capacity these two separate units will put out roughly similar attacks both on the charge and in prolonged combats (Genestealers have the mild edge on the charge due to increased body numbers). The Genestealers however have the massive advantage of Toxin Sacs which against T4 or less units, allows failed wounds to be re-rolled. Statistically this allows the Genestealers roughly 1.5 bites at the apple in terms of rending and obviously increases their wound count over Raveners regardless. This is really huge as it allows the Genestealers to have a bigger impact before they are in turn struck. Raveners mitigate this somewhat with scything talons which effectively re-rolls 50% of their misses against Marines though Toxin Sacs are obviously the better option here (remember, extra chances of rend = good).
Whilst Raveners and Genestealers are both faster than your stock standard Marine and hit on 3's, Genestealers have an extra pip in each of these stats (Weapon Skill and Initiative) meaning they'll do that much better against units with improved WS or fast units (i.e. Dark Eldar). Genestealers have the added benefit of being Troops though being shock units, this generally doesn't mean too much as they are unlikely to live through the game. The main difference between the units is their defenses. They are both T4/5+ and benefit a lot from cover but whilst Genestealers maintain their effectiveness through bodies, Raveners do this through multiple T4 wounds. This is really important for target priority in your opponent - Raveners attract the same type of firepower your important units (Tervigons and Hive Guard) attract which minimises the effectiveness against those other units or your unit maintains maximum efficiency for longer. At the same time, S8+ weaponry and quickly decimate this unit though flamers can do the same to Genestealers.
Conclusion
What we get in the end is very similar units with each being a bit stronger in different areas. These strengths are what you are going to base your unit selection on. If you want smaller units which can get into your opponent's backfield quickly and saturate the battlefield against S8+ weaponry, Raveners are your bet. Not having to spend as many points on these units makes it easier to fit into lists as well (105-140 points for 3-4 strong units are fine). If you want larger infantry units which can benefit from Catalyst and are better Marine killers thanks to toxin re-rolls, go for Genestealers. You really need to be able to cast Catalyst on these guys to get the best from them however to make sure they aren't bolter bait and this can fall apart against psychic defenses or simply overload your Tervigons with Catalyst targets.
Either way, each unit is going to add fast combat punch to your army with lots of rending attacks at a high weapon skill and initiative. If you're looking for more combat power in your army, it's hard to go past either of these units. If you had to pin me down on which was the better unit, I'd go with Genestealers with Toxin Sacs. Those re-rolls to wound and extra chances to rend are huge though Raveners do get re-rolls to hit and are more effective as cheaper squads (i.e. I'd want at least 10 Genestealers).
