The Poem is 'Rags' by Edmund Vance Cooke, and the verse you partially recall is
And if there's no heaven for love like that,
For such four-legged fealty—well!
If I have any choice, I tell you flat,
I'll take my chance in hell.
It's actually a pretty gruesome tale, Rags saves the narrator's life twice on the Front and is brought back to the UK with the Regiment, but is 'stolen away'. The narrator next encounters Rags
in a vivisection laboratory, "tacked and tied
And slit like a full-dressed fish,
With his vitals pumping away inside"
Rags recognises the narrator, wags his tail a little and dies.
In case it is of interest how I found it, when I had no success searching your phrasing directly I tried substituting 'heaven' for 'reward', as a likely juxtaposition to 'hell' and 'take my chance' rather than 'take my chances' just because I thought it had greater strength for a closing line. That brought it up on the first page of returned search results.