Another with experience here (DP is from there, so we.koved when we had kids). It's a great place to live with kids, lots to do, great schools etc. It's easy to access the countryside and sea, and not far from Brighton or London.
However, unless you want a painful commute, I wouldn't go for one of the new builds on the edge of town (we have 2 stations- Littlehaven and Horsham. Of these, Horsham has way more trains and faster trains (not sure the LB trains stop at Littlehaven? I know a proportion of the Victoria ones do). You also don't risk getting caught at a level crossing if you are late).
I would spend some time familiarising yourself with the various areas and the housing stock available against what you need, as what is classed as Horsham is spread out (same as TW and other market towns, TBH). We also like to say that a few areas are 'rough for Horsham' - places that locals think are rough, but elsewhere you wouldn't blink an eyelid at them.
It can be a change of pace for Londoners (most market towns are...I am a born and bred Londoner) but, for a market town, it has a lot happening- lots of restairants, bars, an everyman cinema, theatre, regular markets with local produce and so on.
Depends what you are looking for! We have friends in TW, and prefer Horsham... but that is because we were able to afford what we wanted, and managed to find it where we wanted it. Friends currently looking to move to a new property are struggling to find what they need in their price range.