Hi,
I have never used Sienna so cannot comment, I do however use Suntana on a daily basis (I am a Mobile Spraytan Therapist) and can vouch for the solution.
It is the cheapest solution that you have probably found, but dont let that little fact deter you as I very nearly did, (I originally thought it must be a load of rubbish if it's so cheap).
But I PM'd Suntana on here and asked for a sample of their solution. They duly sent said sample (approx 100ml, so a good 2 bodies for that), I tested it on myself and was amazed, it smelled gorgeous, had a great guide colour and lasted ages (bout 13 days, cos I looked after it properly). 99% of my clients ask for this solution as oppose to the other solution I use.
Dont just take my word for it, PM Suntana and ask for a sample of solution, try it for yourself & let me know what you think.
Hope this helps
xxx