Ryan,
I'm a CRE Broker myself, although I really focus more on development and I'm very niche in my brokerage. The 3 types of specialization are different and I'll outline my experience below. Another thing to consider, will you be coming in at the analyst level or directly as a broker? and if you're coming in as a broker will you be assigned to a Senior Broker (who can train you?) Brokerage is a difficult relationship driven business, so it's important to make sure you go into an environment that will be willing to invest time and effort into helping you develop your professional skill set.
1- Investment sales, (this is my focus) This is a low volume high margin business. For example, I haven't closed a deal in over 14 months but the deal I have under contract now is for $25mil+ and will net me a commission of ~$750k (as in repping both the buy and sell side). Its also important to note that within this category there are many product types and economic specializations (industrial, medical, office). Find out a little more about the brokerage and whether they will put you in a product type team, and send me a PM and i can expand on each one if you'd like.
2- Landlord rep, this can be extremely lucrative and tends to be more high volume low margin. Basically, the name of the game is finding property owners willing to let you lease their asset for them and if you do a good job and maintain the quality of your work you basically can create recurring revenue into perpitutity. Again many people specialize by product type in this sector as well so it just depends what the firm and what you want to do.
3- Tenant rep, can also be lucrative but is the worst option in my opinion. I look at it as essentially the worst of both parts of IS and LR. Low volume low margin. You're essentially selling a tenant on a deal only to receive a fraction of what you could (in most cases) for selling them the property. It's essentially the same amount of work as investment sales for less money, with less likeliness to lead to more income from the same client.
I hope this was helpful and id be glad to talk over it a bit more if youd like to in a PM. Of course every sitaution is different these are just my opinions based on my experiences. This was hard earned knowlege for me so I hope you use it well!
JR