Mycoplasma genitalium has less than 600 genes. We have something like 30,000. So a ballpark answer might be "at least 50 times harder". I expect it would be very much more than that, as a free-living microbe has much simpler interactions with everything around it, while a neuron can have connections to thousands of other neurons. Neurons are also much bigger, with more physically complex stuff.
Thinking in terms of uploads, it might not be necessary to simulate all that in order to duplicate whatever is important about its function. If you don't know what is important about its function, then you may have to brute-force it at the highest level of detail you can manage, at least until you discover what is important.
ETA: Also, neurons are faster. The time step of their simulation was 1 second. For neurons transmitting electrical signals, you'd need somewhere below 1 millisecond resolution. So there's another factor of at least 1000.
So a ballpark answer might be "at least 50 times harder".
The "at least" part seems wrong to me. Cellular differentiation works by deactivating some genes more-or-less permanently and by sequestering deactivated genes in densely packed regions of chromatin that are inaccessible to transcription complexes. (This is a one-sentence summary of an absurdly complex biological process. You have been warned.) Understanding the functional molecular biology of a highly differentiated cell type like a neuron won't require the understanding of 30...
"A Whole-Cell Computational Model Predicts Phenotype from Genotype" by Jonathan Karr et al.
This paper appeared a few days ago in Cell, and describes a computational simulation of the bacterium Mycoplasma genitalium, conducted at this lab. The paper is behind a paywall, but is blogged about here. The simulation software is freely available from the project web site.
From the abstract: "Here, we present a ‘‘whole-cell’’ model of the bacterium Mycoplasma genitalium, a human urogenital parasite whose genome contains 525 genes. Our model attempts to: (1) describe the life cycle of a single cell from the level of individual molecules and their interactions; (2) account for the specific function of every annotated gene product; and (3) accurately predict a wide range of observable cellular behaviors."
According to an editorial commentary in the same issue, this is the first simulation of a complete free-living microbe.