The other answers were good but I want to go into more detail.
Ferrothorn
First off I'm going to say both are not ment to be tanks but walls but with the right movest ferrothorn can tank where as forrtress cant.
base 133def and 116spdef isnt bad at all and it can do pretty well with a sassy nature.
It has access to a good movepool and entery hazards such as spikes and more commonly used stealth rock.
Stealth rock is a good idea to have on both forretress and ferrothorn as it does at least a quatre of nearly all fire types hp.
It also has a 2x weakness to fight and ground which can be covered with high powered stab power whip.
You may have more problems with fighting types but with its amazing ability iron barbs they can loose a good amount of hp when attacking you seen as nearly all fighting moves involve contact.
You can go one step further and give it the rocky helmet with that any Pokemon making contact with him will loose 1/4 of there hp, that paired up with stealth rock and possibly spikes =at least 1/2 there hp gone before youve even attacked.
Obviously you dont need to use entery hazards or even give it rocky helmet, leftovers is also a great item to hold.
Ferrothorn is by far the most popular but each Pokemon work well in there own tiers.
It has a decent atk to add to his good defs as well as decent hp.
This goes great with its terrible speed as you can use its base 20speed to its advantage.
With no ivs or evs at lv 100 with sassy nature its speed is 40.
With that speed the only thing it will be out running is shuckle which it doesnt meet in its tier.
Running curse is a great idea seen as its speed cant get much lower but at the same time your raising its atk and def.
This makes him even bulkier but also gives it a chance to do a lot of damage with moves like power whip or two moves that go great with its terrible speed, Gyro ball and payback.
There are many differant sets you can run for ferrothorn, walling sets ,tanking sets and stalling sets of those forretress can only pull off one.
To add to its walling capabilitys it gets leech seed and has avalability to good stalling moves like toxic or thunder wave and protect.
But its worth considering putting more evs in its hp and spdef or atk seen as its def is already great and contact moves will backfire with iron barbs.
Its also worth considering the move explosion because if your about to die you may as well die in style as well as damage or most defintly kill the oponent in the process.
There is a reason that its in smogon ubers as well as ou.
I think we have enougth here.
here is a good link to some movests that may have not been mentioned in black empoleons.
Forretress
Ill try to make this one a bit shorter.
So he has also made it into smogon ubers as well as ou mainly because of entery hazards.
Forretress has access to all entery hazards.
So toxic spikes is a great move to have and works as a great supportive move in a way.
Stealth rock once again is also a good option and is one of the most commonly used moves and can covers its fire 4x weakness.
As for stat wise, there not great 75hp 90atk 140def 60spatk 60spdef 40speed.
So stat wise it has great def and should be able to take at least 2 neutral hits and even maybe 2 super effective atks as for fire though it can take 1 or 2 it depends on your ability because it gets overcoat which is pretty useless unless your facing weather teams but it can also get sturdy.
its hp and spdef though are not good together any super effective speacial hit will ko him and it wont take many neutral hits.
Thats why I personally perfer ferrothorn but once again foretress can also abuse gyro ball and explosion and do good damage.
A common move seen on forretress is volt switch to scout and to escape .
But the two moves that makes forretress so over used in ou is the fact it can learn TOXIC SPIKES and RAPID SPIN.
This is in caps and bold because I wanted you to see it and I'm geussing most people wont bother reeding this all ive just noticed how long it is.
But back to the point rapid spin and toxic spikes are two moves ferrothorn cant learn and two moves that are seen alot on forretress mainly because not many Pokemon learn t spikes and rapid spin removes entery hazards which could of ruined your team.
But once again back to stats and with maxinum Ivs on all stats with the evs of 252hp 4def 252spdef and sassy nature its stats look like this > 354hp 216atk 317def 156spatk 240def and 104 speed.
so now you have made its not the best hp decent and above average and for spdef, well its still not supperb but its ddefintly not rubbish and with that hp and spdef you should be able to take a few good hits though with this spread you are loosing some valuble def but with 354hp and 317def thats also not bad at all.
Unlike ferrothorn it has better typing and as someone said only one weakness adding to its bulk but forretress can not pull off much appart from setting entery hazards which is ok but now in fith gen with the likes of espeon in ou and xatu avalible in ou can wreck forretress something it cant to ferrothorn due to its offensive movepool.
If you predict wrong against certain teams well you have a problem.
As I said the new ability magic bounce came through dw and has made espeon a very good choice.
I got beet the other day by a infernape u-turning to an espeon bouncing back my stealth rock which 4 Pokemon was weak to it 1 with a 4x weakness.
So in this situation what do you do? your entery hazard chances are gone and youll just have to hope that you can set them up in a later game.
Though if you have zoroak on your team with forretress last it can take out the magic bouncers.
But there are many cons to forretress due to its threats but as ive said if you want a tanking position forretress isnt the best option.
So overall
If you want a tank :ferrothorn
If you want a good wall :ferrothorn
If you want a staller :ferrothorn
and finally if you want an entery hazard guy :forretress
