This is a great question, and a difficult one! There is no simple answer, but I will do my best. (Also, it's worth noting that I am not a professional of any sort myself.)
I can understand both sides of professional opinion in this case. Wanting your child to strive for their highest potential is good. So is giving them the easiest access to life. My parents really did not push me (at least in the area of mobility aids.) There were times when my dad pushed me to write faster, so as not to need extra time to take my PSATs. In that instance, his pushing was a good thing. It turned out that I didn't need the extra time and could complete the test without it. There have been other times, though, where I have been pushed to complete things that were physically impossible for me. In that case, pushing is very damaging for a child's self-esteem.
In the end, given your child's very young age, and the fact that they cannot advocate for themselves and what they would benefit the most from, it will be up to you (and your partner, if you are co-parenting.) I absolutely respect doctors and therapists and their opinions, and I am in no way asking you to disregard them. However, a doctor might see your child once every six months. A therapist might see them for a couple hours a week. You see your child every day.
So, use that. Observe your child and how they move. Consider questions like, is your child able to use their arms? Are their arms easy for them to move or are they affected by high or low muscle tone? Are they able to sit up with a minimum of support or do they require more? If your child's arms are not affected by high or low tone, and if they are able to sit and balance reasonably well given back support, I (in my completely unprofessional opinion) don't feel like it would be unreasonable to pursue a manual chair. It would give your child the opportunity to be more physically active and build strength. However, if your child's arms are difficult for them to move, and impacted by spasticity or flaccidity, and if they require more support than a seat belt and a back rest to remain upright, perhaps, consider an electric wheelchair. Also, is your home suited better for one chair over the other? Do you have a vehicle that can accommodate an electric wheelchair if you choose to go that route? All just questions to consider (and maybe you have thought these through already.)
I just went through the process of getting a new chair myself, and I was able to use a chair of the same make/model that I was considering for a period of a week or two, to see if it suited me. That would definitely be something to ask as you move on in the process of getting your child a first wheelchair. It would give them the opportunity to get a feel for it, and you, to observe which chair works for them. If possible, ask about an option like this at any sort of evaluation your child has.
In the end, it really is up to you. You know your child. You're around them and likely have a very good sense of what they can do and/or have the potential to do with time. It may take your child more time to learn to navigate either chair, but be patient. If you have any other questions, please let me know! I hope this helps!