I'd go for a top-tube mounted seat any day. The kid can see forwards, instead of looking at your arse all day, the balance is much, much better, [I remember a bike falling backwards when I stopped for a rest, with a child in a rack-seat] and you will both feel much safer with the child between your arms where you can talk to each other. If you get a top-tube seat, cut off the stupid back-rest thingy. I put a roadie-style aero-bar on my MTB so my two could lean forwards better, but the perfect mod. was a fork with rack-mounts on the blades. I bolted some ally. footrests onto them (I'll give you a diagram if you want) and both the balance and comfort for both of us was great.
i'd prefer him on the back, i wanted to get an adaptor for my trailer, but wifey wont allow it, although its good enough for the dog, but not the nipper, so as a comprimise i'm going for a rear mounted seat.
Hi We had a similar problem and last October we ended up importing a front mounted one for our ten month old son from the US. He loves it and is always smiling when we go out. It is called an iBert safe T seat. Might be worth a try.