My take:
Get Aslan for sure for the interesting chargen and RP opportunities for a purely Aslan player group. The subsector included is an intriguing place as well.
I was a die-hard Vargr fan in my youth, but I like MgT's Aslan better than their Vargr book. Still, there are some fun rules that would make playing a purely Vargr party a lot of fun, with players in friendly competition to be top-dog (so to speak) of the crew.
Dilettante is surprisingly interesting, but it will stretch your concept of what a Traveller campaign can be. Even reading through the various mishaps and evens in the character generation part is amusing. This book takes you way further into the realms of a setting like Star Wars, Tomb Raider and Flash Gordon, where the players are "Important People trying to Save the Realm" rather than Firefly's "Everyman trying to keep his ship from being reposessed". You could easily take it into the realms of comedy as well, with pop stars, sports heroes and misunderstood playboys galavanting around the subsector seeking thrills, fame and the next epic party. ("What do you mean, I don't have launch clearance? DO YOU KNOW WHO I AM?")
If the idea of that sort of campaign interests you at all, grab this book.
Merchant Prince is a mixed bag, and will prove useful if you are running a heavily merchant based campaign. There are tons of rules for buying and selling and determining what's in the box you're transporting. If your players aren't into that level of detail though, and consider the commerce just a "necessary evil" to owning their own starship, you won't need this book. I do chuckle every time I see the picture of that, um, "prosperous" Vargr and his den Darrian slave girls though.
Scouts, though I sincerely wanted to love it, didn't really grab me for some reason.
Something that is a recurring theme in the various Alien books, as well as Dilettante (which is almost an Alien book in its own right) is that they add rules for rewards beyond money. This allows a group of similarly "booked" characters to have adventures that won't be like the standard Traveller adventure. Dilettantes will be primarily seeking Fame (Social Standing) and maintaining (or even losing!) their Fortunes. Vargrs will still seek money, but more importantly they will try to increase their personal power by one-upping their peers. Aslan males will go to crazy lengths for Honor and Land, and females will seek out science and technology while still supporting the wild actions of their males. Is it stereotyped? Sure, but it can still be fun.
In summary, the MgT Alien books (haven't seen Darrians yet, but Pete Nash did us a solid with Dilettante, so I suspect it rocks too) give you more than just a way to add aliens to a standard Traveller game. They really give you a whole new sub-setting to campaign in, or at least an alien sphere to send your players to to show them that not all of the galaxy is a lawless frontier where fortunes are won and lost in speculative trade, guns for hire and mysterious artifacts.