I am not an expert on this topic, and I'm assuming there will be some CPA's and financial planners to give more specifics.
In my situation, I use a SEP IRA which I rolled over from my employer's 401k plan in 2001 when I started my practice. I purposely went with the SEP IRA for ease and simplicity. Through last year, the contribution limit was $50,000 or $51,000. The other limit is no more than 25% of your net operating income can be contributed. Additionally, I knew I'd never need to borrow from it, so that was another reason I leaned toward the SEP IRA.
I'm not too familiar with the solo 401k plans. Since your question caught my attention, i did a quick Google search on "SEP IRA vs solo 401k" and you will find a great starting point in formulating a decision. However, i will be anxious to see more feedback.
I will continually stick with my SEP IRA as it's low maintenance and the annual fee is insignificant. Plus, my wife and I can EACH max out our IRA's as well (which seems counter intuitive to me, but are not complaining).