The 'Grammar Guide' is the shorter, more concise presentation of all the key elements of Japanese grammar. It explains pretty much every grammatical construct in modern Japanese - though not of course every word or expression, and not everything from classical Japanese which is a whole other thing.
The 'Complete Guide' is more meant to replace a textbook course, and presents the same information as the Grammar Guide but with more examples, more detailed explanation, etc, etc. Same stuff, more handholding.
Both Tae Kim's guides use his unique approach to the language based on his (quite good) understanding of the language. It does involve some odd pseudo-english examples to demonstrate how Japanese grammar works that puts off some people. Some of his terminology and grammatical theory is at odds to traditional ways of teaching, but it is all consistent within itself and with the language.
Imabi pretty much tries to compile all the traditional explanations in textbook style on a website. It gets pretty dense in detail at times.
Personally, I went through Tae Kim's Grammar Guide after completing a textbook course, and found the alternative point of view - as well as a few points that hadn't been covered in my text - to be valuable in rounding out my understanding of Japanese grammar. I'd say that could work for you too. I wouldn't recommend the complete guide for someone who has completed a textbook course -- the Complete guide is more for those who are avoiding using a traditional textbook at all.
It doesn't hurt to spend time on Imabi either, although much of it will be redundant with your textbook; in any case it makes a good reference site for you to keep bookmarked.
If you ever feel up to studying Japanese Grammar in the Japanese Language, I recommend Niwasaburoo,
http://www.geocities.jp/niwasaburoo/ (庭 三郎 の 現代日本語文法概説). His 'outline' covers pretty much every important point of modern Japanese Grammar. It is slow going though if you aren't already comfortable reading Japanese texts. Perhaps best in your case to bookmark and check in a year or so.