If you go for the sigma 70-300 try and go for the APO version if you can - its a slightly better lens than the original version. Remember that the sigma is a 1:2 (or 2:1 I forget which way it goes) which means its not like a proper macro prime lens which gets 1:1 - the result being you can't get as close as with a true macro, but you can get some good results. The trick is it has a long minimum focusing distance which - whist nice for not disturbing the insects - can be fiddly to work with at times. Personally I find it a taunting lens for insect work - just not close enough for me - but slightly larger macro and close up work it works really well - just try to keep it on a tripod the whole time!