I suspect that this might be a little out of date, but...
Xan is a good race to play when solo, because its (usually) fairly good defensively. For many planets, its tough to attack someone who has (relatively, *cough*) early firing ships with high firepower (*cough*
) - at the end of the day, the ability of the attacker to kill your fleet due to the light armour isnt as important as the losses he'll take in order to get your roids - Xans tend to be expensive (at least for non-cathaar, and they dont kill ships much), and that means you're a (relatively) worse target.
On the other hand, Xan is a "crash or crash through" race, particularly when attacking. Playing solo means you dont have wide access to scans (at least, prior to #transcendency - is that still available??), and thus you have problems with getting JGP and/or Unit or Advanced Unit scans, making attacking perilous. Monroe suggested attacking with your galaxymate's alliance raids; they usually have access to good scans and scanners, so that's a pretty safe way to go. However, if you're Xan (particularly with Jammers), scanning is a waste of freaking time, so attacking is risky.
Terran is the opposite. Essentially, Terran ships (usually) have the sheer armour to land on virtually anything and the risk of loosing your whole fleet is low. At least, much much lower than Xan, where you can loose your whole fleet and ruin your round really easily. However, due to (usually *cough*) poor initiative and *arrrgh* firepower, you tend to be a relatively cheap target to attack - particularly if you're landing more attacks and getting roids. Fast construction time means that you can quickly gain Amps to scan for yourself, or Jammers to help with fake attacking (really easy for Terrans due to low ship counts). But the constant incoming might get tiresome, particularly if they're kill ships, and it might feel like you're "being pecked to death by ducks".
So, its a matter of preference, really. Essentially, any success you may or may not have depends more on your galaxy than your race choice, but try and pick a race that more suits your style of play. I usually go Xan, but then that means little.
(tell us how you're going?!)