I never tried the Fresh Indulgence Electric but did try their other ranges (Caribean Indulgence & Revive) and my models liked it but LA Tan proved more a popular colour with my clients.
I decided I didn't want to stock more than 1 brand, as solution shelf life meant I would be unlikely to use them all up in time and I'd be throwing money away.
I am not 100% sure on Nouvatan 20% price but the fact that LA Tan do the BOGOF offer usually means LA is cheaper.
All fast/rapid tans longevity is going to be slightly less than having a 'normal' 8 hour solution because the DHA doesn't penetrate as deep in the layers of skin so wear off just that little quicker than a 8 hour solution but they still seem to last approx 5 days so it means weekly tanning can be done with no old tan remnants causing problems.
If your clients are used to the 12% that Electric gave them but want darker then I would advise their first 'new fast tan' (using whichever soluiton you try) should be washed off to get an approx 12% so they can compare the Electric 12% and the newer 12% and then see if they still want darker - if so leave it on for longer next time.
I believe that the colour of a fast tans go up approx 1% every half an hour it's left on, after an hour or so, so for example if an approx 8% tan is reached after 2 hours then at 4 hours would be approx 12% and 4 and a half hours would be approx 13% tan. (Hope that makes sense).
I have not tried Nouvatan's 20% yet either - thinking I might give it a go and see how much of a difference there is between the LA & Nouvatan colour outcomes as LA & Nouvatan were the joint number 1 in my solution trials, I chose LA Tan for price and the guide colour.