I read an interesting article today on BoLS regarding Land Speeders versus Attack Bikes. While I suggest you read the article before continuing here, I will say that it brings up an interesting prospective on the age old question for Black Templar players: Which is the better devastator squad substitute?
Based on the pros and cons listed in the article (not going to restate them as he does a good job listing most), you get the feeling that he is biased towards attack bikes. While I usually take the same stance, I do have to point out that both attack bikes and land speeders armed with multi-meltas or heavy bolters cost exactly the same (discounting the fact you have to take 2 attack bikes in a squadron). So in my opinion, you have to decide based on three key characteristics. These are mobility, firepower, and survivability.
The land speeder has the distinct advantage in the mobility department. Its a skimmer, so it flies over everything it needs to. There is no way to block this unit in, so it'll never be stuck in place whereas an attack bike might find itself in that situation. Not to mention, you can deep strike a land speeder onto the board which can certainly create a surprise for an opponent.
On the other hand, the attack bikes are relentless. They fire both their twin-linked bolters and the heavy weapon up to 12". The land speeder can only fire one weapon after moving 12". However, you can take a heavy flamer to add some punch to the speeder, but this brings you dangerously close to the enemy so careful planning is needed when using them. Because of this, speeders find themselves in one dedicated battlefield role and have to stick to that role in order to be successful. Attack bikes can fulfill multiple targeting roles, whether using rapid firing bolters or tank hunting MM!
Lastly, survivability. Turbo boosting gives the attack bike a 3+ save whereas going flat out with the land speeder gives a 4+ obscured save. Obviously, the 3+ is a better save in all circumstances. Unfortunately, attack bikes are relatively easy to kill with their 4(5) toughness value and 3+ normal save. One krak missile launcher hit (or equivalent) will take out one bike with instant death. As a result, you need to use cover as much as super-humanly possible. Land speeders do have the benefit of having AV 10, but unfortunately, most weapons in the game do have the ability to at least glance the vehicle. This will at the very least, prevent the speeder from shooting that turn. Against enough str 4 shots, a 6 is bound to be rolled eventually. They also suffer from the vehicle squadron rules concerning movement. If a damaged land speeder cannot move, but you move the rest of the unit, that damaged speeder is automatically dead. Likewise, if you get an immobilized result, it automatically dies if you are in a squadron.
Based on this information, I'd suggest using attack bikes. However, if the Black Templars get a similar codex to the 5th edition Space Marines...then using the Typhoon pattern Land Speeder might be a worthwhile option. ;)