I would definitely use the Sparse design. You could go with the sparser design you describe at the end of your email, with 10 questions of 5 items each, but I think you'd do better to be a little less sparse and have 20 questions of 5 items each and two item-views peer respondent.
We've now conducted a pretty definitive empirical test of Sparse and Express using human respondents, something we'll be sharing at our Turbo Choice Modeling event in March (Turbo Choice happens in the two days prior to our main Sawtooth Software Conference: http://www.sawtoothsoftware.com/events-hidden/1839-2018-conference).
Whereas our artificial data showed a slight advantage for Sparse over Express in terms of the validity of respondent-level utilities, with human respondents the advantage is much larger and still in favor of Sparse. We even looked at the ability of Sparse and Express to reproduce the segments you'd get from a more fully-informed MaxDiff (3 item views per respondent) and Sparse reproduces segments MUCH better than does Express.
I don't want to give away too much, because my colleague Megan Peitz will be showing these results in March, but those are the high level results which I think should steer your decision.