jemima- i bought a HAB when my little girl was 8 weeks old, my only regret is not buying it earlier... i cant wait to use it on my new borns in the future.. but in saying that- Eden is now nearly 7 months old and with this years tax return we are buying an ergo... not because the HAB is outgrown but i think it may be a little better to have her on my back sometimes and may be a tad more comfy for longer trips... Eden is a pure carrier babe(been in her pram about 5 times) so is all about grabbing for stuff and being involved... so the HAB isnt working so much for us now for awake shopping trips etc.. but for putting her to sleep in public, i cannot fault it...
So.... my advice is.. get the HAB... then if ur babe is still a carrier lover get the ERGO down the track. U say you dont wanna get both as this is ur last babe... I've heard they bith have great resale value. So id say they are both worth it.