I started my search for a lens by going after the Tamron 90mm Di. It reviewed well, but proved difficult to get hold of. I'm not experienced at buying lenses online and I'm wary of buying high-value items from suppliers I don't know.
One or two sites claimed to be able to supply it for a good price, around the £330 mark. But I sought out reviews of those sites and the feedback was generally negative.
I then broadened my search to include the Sigma range of lenses. They do a 105mm and 70mm macro, but both were priced at around £400. The Sony macro lenses were even more expensive.
My desire for a longer lens was driven by the desire for a solution that allowed me to stand further back from the subject matter. However, as I considered the cost of these lenses I thought about the sort of macro photograhy I wanted to do.
Some of my subjects will be living creatures, such as insects. Being able to keep my distance would avoiding disturbing them. However, a lot of my macro work will probably be with static objects (such as the peas) and in relatively controlled environment.
This is why I opted for the 50mm lens from Sigma. It's not the ideal, but I'm hoping that it will be a good compromise between length and price. Time will tell.