Calvin was an excellent theologian while Luther was more of a preacher.
I suspect that is more exemplifies the differences between the way the Reformed do theology and the way Lutherans do theology.
Calvin was a systematic theologian, his writings reflect that.
Luther wasn't, for Luther what mattered was being true to the theology of the cross and that, absolutely, meant being a preacher of the Gospel.
To that end, the Lutheran tradition has generally been less interested in doing systematic theology the way the Reformed have; instead our theology is is confessional and kerygmatic.
To put it another way, confession and preaching
is our theology. We are less interested in offering philosophical ponderings about things we can't know, such as why if God wants all to be saved and if God has predestined us in Christ to be saved, then why are some not saved (the answer is because they choose to not be saved). Like how does that all work? Well, rationally speaking it doesn't. Now in the Reformed tradition those niggly bits seem to get ironed out through a systematic approach, often by applying reason. To wit, if God is the effective cause of our salvation by predestining us in Jesus Christ, then it must follow that the reason why some aren't saved is because they were not chosen by God to salvation, and thus their damnation follows from the same kind of decree from God--there are those who will be saved and those who will not, and this by the will and decree of God. No question that this makes sense rationally, but does it hold true biblically? Well, no, says the Lutheran. And for this reason the doctrine of predestination is exclusively a doctrine of grace, a doctrine of comfort that we might hold firm in confidence the truth of God spoken to us through Word and Sacrament: That we belong to Him because He chose us, called us, and made it happen--not through some inscrutable act of divine will, but through the visible, outward, external working of God through Word and Sacrament.
Thus we preach and confess, and this is what we believe.
So by presenting Luther as "more of a preacher" and Calvin "more of a theologian", this is less a contrast between Luther and Calvin, and really more of a contrast that exemplifies the very different ways these two traditions actually
do theology. This can be seen, for example, in Luther's Heidelberg Catechism where he contrasts the false theology of glory with the true theology of the cross. One is a true theologian not by speaking of and recognizing the invisible things of God, His glory, His power, His wisdom (etc); but rather one is a true theologian by speaking of the visible things of God, namely His suffering and death on the cross.
This is why for Lutherans the chief article of the Christian religion is not the glory and sovereignty of God; but rather is the free gift of righteousness by the grace of God alone, through faith by which Christ has saved us.
It always comes back to preaching the pure Gospel as pure Gospel.
-CryptoLutheran