I haven't seen any improved varieties of strawberry/lemon guavas. In my own experience:
Strawberry (red) guava advantages: Plant is more vigorous, heavier bearing, fruit keeps a little longer on the counter.
Strawberry guava disadvantages: Smaller fruit, a little more astringent than lemon guava, a little more tart, and the fruit's red color might attract more birds/pests than yellow ones.
Lemon (yellow) guava advantages: slightly larger fruit, softer fruit, better fruit to seed ratio, sweeter.
Lemon guava disadvantages: Less vigorous plant, fruit doesn't keep as well on the counter.
I'd go with the lemon guava, but that's just my two cents from growing both varieties in Southern California.