The answers are somewhat situationally dependent:
What is your current civilian clinical skill set and certification/license: If you are applying to the reserves, and you are not currently qualified at a minimum as a Primary Care Paramedic (PCP) in your home Province, then you cannot apply for Paramedic. You will have to go Combat Medic. If, in the future, you became a qualified Paramedic, you could apply for a Voluntary Occupation Transfer (VOT) to Paramedic.
Is there a spot available: Does the Reserve Field Ambulance you are seeking to join have many Paramedics? Every unit in the Canadian Armed Forces has a set number of positions that are further divided up by rank and occupation. So, each Reserve Field Ambulance will have a set number of positions for both Combat Medic and Paramedic. If the Paramedic positions are full, they will not be able to take any new applicants, and you would have to be a Combat Medic, even if you were a certified/licensed Paramedic - at least until a spot opened up.
Now, all of the Reserve Field Ambulances are at the start of a modernization program, and all of them are supposed to grow (my unit will be doubling in size). Most of the current troops in the units are Medical Assistants, and will become Combat Medics. That means that the odds are good that if you are PCP qualified, there will be a Paramedic spot available. If not, there will be a lot more Combat Medic positions being created in the units.
The training is also going to change - we need our Combat Medics to be able to better integrate into and support the Combat Arms units - that means adding more field training such as the course formerly known as Basic Military Qualification - Land. We will also be looking and adding more clinical skills to the Combat Medics. That said, the new training programs have not been rolled out yet, so I don't have solid details.
Where are you looking at joining?