Basically, MDF is less prone to movement(expansion/contraction) due to seasonal temperature changes than solid wood. The construction of a raised panel door necessitated the panel be installed without glue/fasteners to allow for that seasonal movement. An example of allowing =for that movement in a dining room table is when the builder makes slots for the fasteners between the top and base instead of locking the top to the base with screws/glue.

However, most MDF will retain moisture when in a wet condition----standing water is a flooded basement---for instance. There is a possibility, IMHO, that dripping water off the counter could land on the panels and seep into the groove made for the raised panel. Enough of that and the MDF would swell, causing the rail to crack.

The type of wood used also has a bearing----oak is less absorbent, but looks terrible(IMHO) painted without filling the grain. Maple is good, poplar absorbs moisture more easily.

It boils down to the climate and uses in your kitchen to determine the construction materials---as well as your wishes. Sounds to me like you'd rather have the wood---and real wood raised panels for the same price as MDF panels is a good deal----or the MDF panels are way overpriced.

While MDF moves less than solid wood, it moves in ALL directions.
Wood only moves in 2 of 3 axis.
Wood panels are routinely fastened in the middle of each end to prevent rattle in higher quality work (there are also little elastomeric balls that can be placed in the grooves to help prevent some rattling).

In most parts of the country it's common practice to use MDF panels if the cabinets are to be painted.
It looks better painted than wood, especailly oak.
Water damage is highly unlikely. Most likely they'll still be looking good when the cheap Ikea's are in the landfill.

Wow, thanks guys for this quick advice! Although, I must confess that I might be even more confused at the moment. lol. Would it make a difference if I were to say that the part of the country we're talking about is NYC and that this base sink cab unit will be situated in a relatively small kitchen (11 x 15), near a large gas range and adjacent to a dishwasher? I'm not sure of the wood the cabinet maker is using, but I know it is not oak.

Painting protects wood or MDF better than other finishes---due to the thicker consistency when it dries.

There are so many variables in this example that it is highly unlikely anyone will give the correct advice for your individual case.

For instance, the gas range will create a lot of heat if the oven is used a lot. The dishwasher will create a lot of steam when opened. The amount of heat/steam and its effect on the cabinets and doors will depend on the frequency of use of the appliances.

That means , basically, if you are in doubt, the MDF panels will probably be the best option as they will be less affected by all the variables.

Personally, I think you should have the cabinet maker make all the cabinets---unless he is a hack, his cabinets will be better than the Ikea.

Actually IKEA cabinets are made by Snaidero and are a very well made cabinet. I would go with the MDF for all the reasons above, plus, the wood doors will almost always crack the paint finish at the joints. Does he have a door so he can match the white and the sheen to the IKEA cabinet? Are you having the sink base made because you afraid of the weight of the sink or because of size? The weight will not be a problem (you can build a cradle if needed to support the sink) size is another matter.
good Luck !

Yes, Tom, I'm having the sink base made for a Rohl Shaw's 30" fireclay sink. I'm aware that Ikea has a base for their own 36" farmhouse sink which I imagine could be retrofitted for my application. However, I just don't have the space for a 36" sink base! Thus, the base I'm having built is 31 1/4" in size. Unfortunately, the cabinet maker refuses to work with me on the color. Says he intends to use Fine Paints of Europe bright white . . .period. Although this does not make me happy, I am tired of struggling with all the project details with him. (I sense that my job is a low end, low priority order in his high-end practice).I figure that, if the match with my Ikea cabs is so far off as to be unbearable, I will have my painter repaint it.

Meanwhile, thanks for the words of support for Ikea kitchen cabs! Since I have already purchased them and have had them installed since early February, there's no turning back now. So far, I've been very happy with them and have no complaints.