As kids we all survived without jumpers and exercise saucer so there not necessary but definitely a luxtury that i'm glad i have. My daughter used the saucer much more but i like the over the door jumper because my daughter would bounce along and be content while i was able to accomplish cooking and a shower. Its easy to transport.
no it is not necessary to have them both but i will say that my son loved his jumper. i would take the exercise saucer and if he really likes it then great if he seems to not like it or seems to get borde with it then i would get the jumper.
I also have a 4 month old daughter. She is playing in her Baby Einstein exercise saucer right now. She LOVES it. My neighbor gave it to me. It helps strengthen her legs, back, and neck. It also allows me to get some things done while she's playing in it.
I say both are good ideas. I had both items and believe me, they were definitely used. If you are looking at budget-wise options, go ahead and get the exercise saucer from your cousin and keep an eye out on the Fisher-Price Jumperoo for when it might go on sale. Or go shopping to a second-hand store and see what they have to offer. I sold my old stuff to one of these places and I know that they are particular about what they buy. You can be sure that it's going to be in good condition. I also purchased a bassinet from it and it was a great purchase. Good luck!