Mark,
As mentioned if you're spending a fair amount then I'd consider a complete soil substrate over the CaribSea products.
ADA Aqua Soil Amazonia is excellent, but avoid the type II version as it causes a lot of issues with clouding/dust etc. Even Jeff Senske (basically ADA USA) has given the same feedback.
Also consider Columbo Flora Base, Oliver Knott's Nature Soil, and TMC NutraSoil (released December 2010). These are designed to be used exclusively where the ADA Aqua Soil is designed to be used with other ADA additives i.e. Power Sand, Bacter 100, Tourmaline BC, Penac P/W etc. However, you will get good results with Aqua Soil alone.
One side effect of some soils is that they release ammonia in the first few weeks (especially ADA Aqua Soil Amazonia).
Have a scan through the threads in this sub-forum to get a better idea.
One drawback to soil-based substrates is that they tend to break down over the months into a 'mud'. This is fine if you are keeping an aquascape long-term with little plans for uprooting plants and moving stuff around. However, if you like to 'tinker' with your plant positions and hardscape (that many of us do, especially newcomers), then this can result in major clouding issues, and potential ammonia spikes, leading to algae and 'dirty' water.
These days I'm actually a fan of inert substrates, or a nutrient-rich base layer topped with an inert substrate. It's much lower cost, and re-scaping isn't as much of an issue. My latest Iwagumi tank is plain 1-2mm sand, for instance.
Anyway, plenty of food for thought no doubt. You'll hear lots of varying opinions and experiences on here, which is great! At the end of the day, if the other components in your set-up are ideal (light, CO2, nutrients, filtration, circulation), then substrate choice isn't actually that important!