Better to ask this question......
Is the new engine a factory engine ? Factory engines have a reputation of lasting many miles.
A rebuilt engine is only as good as the rebuilder. That is, the parts they used and who/how they rebuilt it.
IMHO a 600 dollar rebuild is not a full rebuild. Machine shop costs alone would reach more then 600 for boring, decking, crank grinding, head and valve work etc.
The other factor is simply how much you want to invest in your Jeep. Only you can answer that.
Believe it or not I have close to $6,000 in my stroker engine. But then little of it is factory Jeep.
This will be an entertaining thread.