This can be a bit tough as Attack Bikes and Land Speeders are different unit types - ones based on an infantry statline and the other is based on a vehicle statline so comparing them isn't exactly easy. We can run some mathematics to see which is more durable (which have been done lots of times on the Internet so I'm not going to do here - little homework for you if you want :P) and the end result is Land Speeders come out ahead - barely. Let's break down this analysis though without actual numbers.
Land Speeders are virtually immune to light arms - S3 doesn't hurt them at all and you need a lot of S4 shots to reliably put through suppression damage on AV10. Even then, only S5 has a statistically significant chance of dropping a Land Speeder and it doesn't start to get close to 'good' until you're looking at S8+ like with most vehicles. Obviously S5-7 is more effective than AV10 compared to other vehicles but it still requires a lot of shots (which most of these weapons have anyway).
Attack Bikes however can always be wounded. They are tougher than your average Marine with T4(5) but even S3 is going to be able to hurt them and they don't like seeing poisoned weapons which bypass their higher than average toughness or weapons with AP3 or better to bypass their better than average save. With two wounds they are comparable in toughness to Land Speeders but with only T4(5), they can be instant deathed relatively easily by S8+ firepower - and we know that's common to deal with vehicles like the Land Speeder.
In the end, both units are of a similar durability - they both die to S8+ weapons relatively easily with Land Speeders having a greater durability edge but are easier to suppress.
Army Inclusion -
You can't really decide based on durability then if you want to include such units - they are too similar and although they have different durability strengths and weaknesses, fine combing over them isn't going to net you the best result. Rather, looking at how they fit into your army list will.
At their core, Land Speeders & Attack Bikes are fast melta delivery systems and this is what we'll be focusing our comparison on (there are other options for the Land Speeder in particular but we'll discuss them later). Attack Bikes are more efficient at 50 points each compared to a Speeder's 60 but you get the extra movement options with the Land Speeder and that tiny increase in durability. This is one of the main factors you need to determine when including Speeders or Attack Bikes - how they move with your army. This is also something not included in the durability comparison - we assume the unit being shot can always be shot but Attack Bikes are a lot easier to hide.
With this in mind Attack Bikes work really with with mechanised lists where they can hide behind Rhino/Razorback/Predator hulls until they need to fire - this gives them optimal working environments and if we use the V- or Tri-Rhino movement tactic, well they're firing to full effect with the opponent having limited options in return. There is also the juggling of firepower your opponent must accommodate if there are both instant death infantry targets and tanks which require his S8 firepower - it's all nice and well when Attack Bikes can be shot down by S3 firepower unlike Land Speeders but that's often inefficient. If the opponent is then throwing missiles at the Attack Bikes, they aren't going into the other vehicles. For this reason, Attack Bikes as squads aren't great inclusions in foot lists (i.e. Bikers) due to their ability to be singled out by S8 firepower which doesn't have other targets in such lists.
This same saturation can be brought forth for Land Speeders as well however - they are literally vehicles so including them is going to cause saturation against such weapons. The important part here is Land Speeders are more mobile and capable of shooting over Rhino hulls (cover can sometimes not happen however). The trade-off here is better ability to bring firepower to bear (harder to block, can move over terrain, shoot over things, etc.) but without the ability to hide as easily (more reliance on terrain for this). This can be mitigated by the ability to reserve and deep-strike though ultimately the Land Speeder is more capable of being shot down by your opponent earlier.
The Ultimate Question - which one?
In the end it's really a matter of which army you're running - Space Marines or Blood Angels? Space Marines are generally going to be better with Land Speeders and Blood Angels with Attack Bikes if you're looking for a melta delivery system outside of a metal box. Why? Both units pay for their mobility and Attack Bikes really shine when they can move with Rhinos/Razorbacks and shoot. Space Marine vehicles cannot move 12" and shoot (thus losing firepower) which negates part of the Attack Bike's positives (mobility). Land Speeders on the other hand are more capable of operating interdependently with that little bit of extra durability and movement options. On the other hand, Blood Angel vehicles are fast however which means they can move and shoot 12" with the Attack Bikes operating at peak efficiency.
That being said, this isn't a black and white answer. Blood Angels can still get use out of Land Speeders and Space Marines out of Attack Bikes - their roles will just modify slightly with what is needed. Land Speeders in particularly are capable of being more than just a melta delivery system - they have several weapon loadouts which are effective (i.e. Typhoon or MM/HF) which provide beyond the "I'm a Fast Melta!" tagline. The above is really where such units fit nicely and are able to work the best as a cohesive whole - Attack Bikes in Space Marine armies might have Rhino babysitters to shuffle them around or can use such early game to prevent shooting at them and split off when they can use their mobility to their advantage, etc. It's all about what you want the unit to do for the army and how it will do so. Just remember the slightly different strengths and weaknesses of each unit and apply that in list building and game-play. The ultimate answer really is they will often both work - as long as you play to their strengths.