Best advice is probably to get some samples beforehand. Coincidentally, I ordered a random selection of the ranges I wanted to take a look at from both Baccus and Pendraken. No problem at all. Both great companies to deal with.
Having painted those samples, my personal sweet spot would be 10mm. Like Fred I'm intrigued by the detail you get while the models are still of fairly(!) realistic proportion. Personal preference, though. I truly admire the wonders others have done in various scales, and I've tried my hand at a good number of ranges – but if painting doesn't feel comfortable, I prefer not to enforce it. So, better see for yourself what you can manage without missing the fun.
That said, if you haven't painted anything smaller than 28mm figures, you should find little issues switching to 15mm. There's a new and quite comprehensive range for the 'Punic Wars era' on the market, done by Andy Cooper of Westwind Production/Forged in Battle (
LINK). Perhaps a third option?