Why not write a shortlist of brands you've read good reviews about and either contact them for samples or just buy their smallest bottles to try out on yourself/family/friends before the Sienna X runs out?
It's hard to find a brand that suits everyone and I don't see a problem with stocking more than one brand if you have the client base to use it. You see, everyone has different skin tones and also different preferences. For example, you don't find LA Tan dark enough, but I've been trying it out recently and find the 14% Cocktail gives a lovely dark but natural looking tan on myself. You said you liked Sienna X, but I find it too orange, so our clients are going to all be different too. I'm not suggesting you stock lots of brands, but if your stuck between 2 maybe give them both a go?
For what it's worth, here's my views on those I've tried:-
Crazy Angel - reasonably priced, gives a nice colour (more golden brown though). 9% for brides or pale skin (don't bother with 6%), 13% a nice darkish tan, 16% for darker skins. Express Tan works well too. You leave it on different times depending how dark you want it. (Been using this brand for about 7 years now).
Moroccan Tan - nice colour, but I had inconsistent results at times. rinse off after a shorter period of time and then allow it to develop (won't look that dark when you rinse it off though). Quite expensive.
White to Brown - found all percentages much too light.
LA Tan - early days yet, but so far I love the colour of 14% Cocktail! In my opinion it looks like I've just come back from a hot holiday (I Tan quite well) and doesn't look like a spray tan at all. I'm finding it lasts well too. Only complaint so far is that the time before last I didn't exfoliate before hand as I was in a hurry and also didn't moisturise often enough after the Tan and the fade off was awful! My skin was very dry though. The Fast Tan was also good. Very reasonable priced too.
Sienna X - too orangey and fade off not good. Quite expensive too.