I don't know what's best about replacing them but with your current heaters do you know you can turn up the input to high and the output to low? Then hopefully there will be some more heat left in them for the evenings. We used to turn the output down quite low overnight and turn them up every morning, then turn them down again before bed, a bit of a faff.
My dh managed to persuade our old church to replace their ancient night storage heaters with new ones and it was much better. Unfortunately they went a bit more towards the cheaper end of the market rather than putting in the best ones so it could have been better if they'd spent a little more for the better heaters.
The new heaters were miles better than the old, if you were to change them for new versions I would recommend not scrimping on cheaper heaters, but put in the best ones you can afford as it will make a difference, they are also more controllable and smaller than they used to be.
We used to have the old brown/cream bulky things in our old house before we moved to our current house with radiators 
. To dry clothes that didn't go in the tumble dryer I would load the clothes horse and stand it in front of, but not too close to, the heater in the passageway as there was plenty of space, do you have somewhere you could do that?
Also, you could look at the new night storage heaters online to see what's available now, means you don't have to talk to high pressure salespeople. I think Fischer Future Heat are good at explaining what you can get, we had one of their leaflets through the door a few months ago and I was surprised at the stuff on it, but I don't know how good they are as have never used them.