After New Year the BTS fares increased more than 20% at least on the longer rides: a trip that cost about 40 baht is now about 50 baht. As a result, the minimum 100 baht top-up won't take you very far. I was caught by this change and I was at first totally baffled why my loaded credit was not enough to exit the station.
If you intend to use the BTS more than a little it may be a good idea to top up at least 200 baht each time. This fare increase also means that taxis are even more price-competitive than before. If two people are riding, a taxi is most probably cheaper than BTS. However, a taxi may get badly stuck in traffic jams, so you risk the travel time being much longer than on the BTS.