The best thing to do is to take a look at the included collections, and the sample content of each collection.
First, the full collections consist of two parts: standard and extended. To get all the samples of the full libraries both parts need to be purchased. The full collections have a large number of articulations that are not included in the SE libraries. Also, the full collections have samples for every half-step, while the SE instruments have samples for every whole-step, and use transposed samples to fill in the missing half-steps. In particular, the full libraries include many dynamics patches that can be of great use that the SE libraries do not have (generally, the dynamics patches are in the extended portion of the full libraries).
The SE collections use different terms compared to the full libraries. For the SE libraries, instead of using "standard" and "extended" to describe that include additional articulations for the same instruments, in SE, the additional articulations are called "Plus".
As far as what to start with, it all depends on what you will be seeking to write, and/or the level you wish to start out at. If you do start with SE, you will get a discount towards the "extended" portion of the full libraries. It is not uncommon for people to start with SE, and as they become familiar with the VSL world to then start purchasing full libraries. One common approach has been to buy SE 1, 2, and possibly 1 Plus and 2 Plus, and then add AP Strings for the first full library (or depending on main interest possibly the full chamber strings, or other strings library instead of AP Strings).
There are others, with more financial flexibility, who do start out with one of the large bundles of the full libraries. In general purchasing both the standard and extended portion of the full libraries has major benefits, as the extended portion of the full libraries really help in creating more realistic sounding demos.
Whatever you do decide, once you get familiar with the basic free VI, adding VI Pro would be strongly suggested. Also, many find either MIR Pro or 24 to be of great use (the use of MIR requires the purchase of VE Pro). The Vienna Suite could be another possible option.
In any case, try to figure out as much in advance as possible, particularly what you wish to accomplish, as your writing and overall goals will help you determine what purchases make the most sense, and whether a large or more modest purchase is the best starting choice.
There is much to learn when starting in the VSL world, though IMO, the VSL sample world is outstanding and allows for great flexibility in one's writing styles.
Best wishes in figuring out what you need. You will find many users on this site to be most helpful.