I've owned both. I happen to live in Petaluma, and have had the chance to own so many different Mesas. Like so many said here, the two amps are very different and, in the end, it is about your style, sound and playing them both. As broad generalities, the Lonestar is a more elegant and refined amp. General consensus is that is has some of the best clean tones and best reverb Mesa has ever produced. The amp is targeted at the clean to medium gain/oredrive players. that is the market it is shooting at. the Express is a lower price point model, designed to cover a wider palette, with more gain and distortion, but not as refined or stylized as the Lonestar. My personal opinion is the Express is the latest "iteration" of the Dual Caliber/DC, Nomad and F-series of Mesas- lower price point, more crunch, but not Recto, a little less personality and uniqueness. I say that in that the Mark series has a unique tone and vibe, as does the Rectifier Series, and the Lonestar. Each of those series has its fans and immediately evokes the iconic defining tone of that series. the Express doesn't.
Not to say the Express is bad. It is just designed to be more of a jack of all trades, master of none type rig. It never really clicked for me, and I had bought it just as an extra/backup amp. But, it didn't have any striking tonal vibe in any direction, and I let it go. The Lonestar was gorgeous sounding, but didn't have enough gain for me. The Lonestar clean tones were the best I have heard, equal to a Bogner Shiva (**** I wish I still had that amp).
So, start with your uses, styles and tone, then go play them both, like everyone said.
good luck,
Brent