I have heard pros and cons of both, but am curious what everyone thinks.

I currently have a 40gal tank, whirlpool. I hate it, but it was in when house was purchased and I have already upgraded it to the new thermal coupler for what it is worth. There are 3 of us living in the house, with a fourth due soon and I have 2 bathrooms.

The 40gal tank just doesn't seem to supply enough hot water for back to back showers and this morning, my 3y/o was taking a bath, while I took my shower and my water got cold. She had maybe 1/4 tub of water and I take, maybe a 5min shower. This tank isn't but maybe 3-4 years old, if that.

I have also thought about adding some radiant heat and I see where many recommend a hot water tank for that, which brings me to my questions.

1) Will a wall mounted, gas, on-demand unit supply hot water for multiple functions at once or is that not what they are designed for? For example, can I take a shower, my daughter a bath. What about a shower and running the washing machine at the same time? Currently, that doesn't work well!
My understanding is that the on-demand units aren't great at providing hot water to several items at once.

2) If the on-demand units aren't good at several items at once, can they be plumbed to work in conjunction with a hot water tank? Or is that like I figure not real efficient and a waste of money.

3) While researching radiant floor setups, I came across a hot-water tank that was about $3500, forget the brand, but what makes one of those worth $3500 vs. the $400 units? It was 90% or 95% efficient, but even if it cost nothing it would take 8 years to recoop the cost over my current unit.

4) Is 40gal tank style big enough for 4 people or should I look at a larger unit? Or maybe even just a 2nd unit and plumb the showers to the 2nd one. It wouldn't be hard, as everything is being converted to pex.

My thoughts are....get either a bigger tank style or an on-demand unit. I have thought about trying to use the current tank for a radiant system, as I don't think it's bad, just that it might be having a hard time keeping up.