Answer
$^{205}Tl$ is more abundant.
Work Step by Step
Assuming their atomic weights are approximately equal to their mass numbers, with x being the first isotope's abundance (meaning $1-x$ is the second one's abundance):
$203\times x + 205 \times (1-x) = 204.4$
$x=0.3$
$^{205}Tl$ is more abundant.
