Depends how you run them.
Blasters are safer, and leave you less exposed to assault rapid fire when you roll badly on your 2d6 assault move. That being said they are not the most reliable of antitank weapons, but the reavers mobility lets them target side and rear armour. It's also worth noting that they have a larger threat range (30") when coming in from reserve. They also cause instant death on T4 models (Crisis suits, Broadsides, Wraiths, Nobs, Khorne Dogs, Screamers, etc), in addition instant death means they ignore FNP on T4 models. They wound T5-6 on 2s (Riptides, Marine Bikers, Nob Bikers, Daemon Princes, etc). They wound T7-T8 on 3-4s (Talos, Great Unclean One, Wraith Knights, Wraith Lords).
Heat lances on the other hand are very reliable AT within 9" (though in practice due to not wanting your special weapons at the front of your squad due to wound allocation this is more like 7-8"). Melta and AP1 make heatlances a real terror weapon, you can really use this fear to control your opponents movement. Move a squad of reavers within 18-21" of a tank and that tank will almost defiantly move away from them next turn. This "damage without doing damage" can be a very powerful tool, as the reavers can easily go after another target if the tank moved out of range. The downside to heatlances is they can often leave you exposed especially if you fail to destroy your target, or there are flamer infantry in transports near by, a bad 2d6 assault move and you can easily get assaulted, rapid fired or flamed.
In conclusion heatlances are a specialised anti tank weapons that need to be used aggressively, whereas blasters are a less risky option that can engage a larger variety of targets effectively. Both have their place.