I have been colouring my hair wild colours for years now.  Its fair to say that I have some experience with hair dyes.  So these are MY OPINIONS about these two dyes.

                                              Manic Panic            vs             Special effects

Bright colours                             Yes                                              Yes
Long lasting                        No (1-2weeks)                            Yes(3-4 weeks)
Texture                                 Thick Paste                                 Thick Liquid      
Range of colours                        Many                                          Many
Price                                            $$                                                  $
Availability                            easy to find                                   harder to get

I've used manic panics ultra violet and electric blue on my hair and my boyfriends' hair.  For both of us, it was bright for a couple days and then began to seriously fade.  I know a few people who swear by manic panic; but it just doesn't work for me.

Special effects works great.  I have used blood red, deep purple and atomic pink.  These are very bright and long lasting hair dyes.  I would certainly recommend special effects to anyone who wants to use a semi-permanent hair dye.