I am considering buying either Flash Extractor or a DDI USB Add-on? (I currently use a DDI so I am familiar with the software).
Does anyone have any strong opinions either way?

These tools accomplish different things. You need more research.

They are two completely different products.

Flash Extractor - focused solely to recovering flash media from dumps.
DDI USB Add-On - adds the ability to mirror USB hard drives (particularly WD) natively

What is it that you are want to achieve? How many flash recovery projects are you currently outsourcing that Flash Extractor will do? How many USB projects are you currently outsourcing that can be handled with the DDI USB Add-On?

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Luke
Recovery Force Data Recovery

I can see from both your replies that I need to do more research. I must admit I assumed that they both did a similar job but this is obviously not the case. I am at the early stages of expanding and just wanted to get some feedback.