Details.
Fivelac is an updated version of Threelac and has some extra probiotics plus a pre-biotic food source , but before Fivelac came along lots of our customers said Threelac was the best thing that ever happened to them, so worth trying Threelac first and save yourself £5. You can alway come back and try Fivelac later if you wish.

Should You Make the Switch?
This is a difficult question to answer.  It's best left to letting you know what people have experienced.
First of all some customers have found Fivelac a bit more aggressive than Threelac and have experienced harsher die off symptoms ( Headaches, flu symptoms, feelling a bit fuzzy or grotty). This may be due to Fivelacs extra probiotics having a stronger effect on candida and the body not quite being able to detox the toxin release from the dead candida organisms, so if you haven't tried Threelac then it might be a good idea to start there. 
If you have tried Threelac for a couple of months and find that it's helping but that you need a bit of a boost then it might be worth the switch to Fivelac as quite a few customers who were a Threelac maintenance does for a fairly long term (3 months) found that Fivelac removed the last of their symptoms.

So What do the manufacturers say ?
The Ultimate Defense Against Occasional Yeast and Fungus Overgrowth
FiveLac is a nutritional-food supplement composed of five live strains of good bacteria: Bacillus Coagulans, Bacillus Subtilis, Enterococcus Faecalis, Lactobacillus Acidophilus, and Bifidobacterium Longum.

FiveLac is the culmination of many years of scientific research and is designed to provide beneficial intestinal micro-flora. Balanced intestinal micro-flora has been proven to remove foreign bodies from the intestine and thus fight infection such as yeast and fungus.