Razordons vs. Salamanders is a difficult question IMO.
The problem with Salamanders is: If they hit they do a lot of damage, but in my experience the problem is that they have only one attack. So let's do the math:
Both die rather quickly so I'll ignore melee for now. This is only ranged damage.
Salamander:
One attack, 66% chance to hit, 66% chance to wound. That means ~44% chance of doing _something_ to a unit within 8".
If you take Handlers for 40 points it is 12".
Even someone with an ideal save has only a 50% chance to save against those, that's good. So let's assume a 4+ saving enemy like Ironjawz and you get around 83% of doing damage with your hits.
If you hit you do D6 damage and then have a 50/50 chance to do D3 more. So you average around 4 damage.
All in all that means that a Salamander hits in about half of your rounds, doing 4 damage. We end around just below 2 Damage per round on average if I didn't do something wrong there.
Razordon:
rerolls hits of one if you have Handlers near, and rend -1 if the enemy is close (6"). That attack of opportunity is nice but only 50% chance, I'll ignore it for now and take the reliable stuff.
2D6 attacks (average 6), 33% chance of doing _something_ (66% hit, 50% wound) to a unit within 12".
Someone with a +4 save will save half of it, so you end up with around one damage per round on average.
Now, that sounds like the Salamander would be better judging from pure damage. And in melee the Salamander also wins (same number of attacks, but 3+ hit instead of 4+ and it has rend), no question.
But the 12" range and the fact that the higher number of attacks doesn't make it that much "all or nothing" makes the Razordon better IMO. Razordons are also less reliant on their Handlers, because you can get re-rolls of hits also from Slann or from Astrolith Bearers. I play two and I normally don't play Handlers. They seldom do less than 2 damage a round for me.
In any case you should play more than one. two or three is pretty good IMO.
Bottom line: Most of the time Razordons are better IMO. They also look better.
Against well armored enemies Salamanders are better because of rend and mortal wounds.